OP said his costs were fixed.
It depends how long your contract is for and how much time you want to spend figuring how to mine. Then there is the the question of how risk tolerant you are.
The first thing you should probably do is go to Bitmain’s website. Using the specs for their S9, L3+ and D3 miners, plug the cost, speed and other info into their respective mining calculators to see if you will have the data center space long enough to earn back the cost of the miners. Right now I think the the order of profitability from high to low is D3, L3+, then S9 or T9.  But that is also the order of instability (high to low) as far as profit margin volitility goes. 
Make sense?
To give you an idea right now, 150 Th/s on an S9 SHA 256 miner gets you about $200/day.
1000 Mh/s (2x L3+) gets you about $40 - $60/Day. 
But no matter what miners you choose, count on waiting for a batch to be released, then after you pay, it will be 1-2 months before they ship to you. Markets will change between purchase and delivery and as you are mining. So don’t expect to get as much profit as the calculations say you will.  But as I said time is likely to be your biggest limiting factor.
 

Are you still in business?

With little or no notice.

Viabtc has a massive BCH hashrate when it’s more profitable. Bitcoin.com is better on BTC.
I have found you have to move around as conditions change. No one pool is the best all the time. At least not from a profitability standpoint.

Could not tell you. I left when it stopped being 120%.
Which happened very quietly.

At Viabtc now. They switch between BTC and BCH depending on profitability.
Past 24 hrs. BCH has been 169% more profitable than BTC.

EDIT: The Pool has found 44 blocks in the past 6 hours.

Truer words never spoken. Hate on Bitmain, and they often deserve it, but no one else has really come up with a more profitable magic money machine yet. And given how long the S9 has been on the market, I’ll bet they already have a working  replacement. I bet it’s mining away in their HQ right now.

I ordered an S9 13.5TH on 7/8/17 that was promised to ship between September 10-20.
UPS says it will be delivered in the US Friday, 9/15.
But don't recall when I saw the 14TH sale.

Sadly no one I recommend without reservations except Bitmain Warranty in Denver.  But in most cases you need to purchase your miner from them in order to get hosting.  If you are seriously looking for someone to host your miner pm me.

To be honest I have only had Antminers for the past two years and never purchased one of these newer Avalons.  But in general the three most common barriers to using btc asics at home are:

   1. Electricity - do you have enough watts and amps to power one, let alone three.  And are the circuits where you need them.
   2. Noise - Don't know about these, but the noise from an S9 is deafening.  Where I live there are no basements and it gets too hot in the garage.  So I have no where to put these things where my family would not kill me.
   3. Heat - you need to be able to exhaust the heat outside and provide relatively cool air to the intake side of the fan(s).  That's not always an easy or cheap environment to create.  It depends on your house.

So if you can handle those three things go for it.  I mined in my office and still do with gpu rigs.  But once the ASICs got to the S5's they were just too power hungry and hot to keep in the office.  I moved to a local data center for 18 months.  Then the electric rates were too high.  This was when btc was stagnant in the $200-$260 range for a long time. So finally I moved them to Washington State.  They have been there for 18 months or so now.  Good cheap power and it usually does not get too hot.
Anyway - my $0.02.

Same miner (L3+), want to mine the most profitable SCRYPT coin but the instructions on the main page are confusing.  They don't explicitly say this is a profit switching pool, except between algos.  Instructions on the pool page say;

YOU MUST ALWAYS set your currency in your password field: c=<SYMBOL>-

But will I mine the most profitable Scrypt coin if I don't list a coin in the password field?  Just like @isukabtc has above:

stratum+tcp://scrypt.mine.zpool.ca:3433 -u (btc address) -p x

Like just use the field for "x" or "123" or set my difficulty?  Or is this NOT a profit switching pool?
I'm thinking it is, just does not exactly say it....that I can find.

I believe the most efficient ASIC miners right now from a profitability standpoint are the Antminers, D3, L3+, S9, T9 in that order.
But only the S9 and T9 mine bitcoin and can be discussed here in the Bitcoin Mining Forum.
The D3 and L3+ mine altcoins and need to be discussed in the Altcoin Mining section of the forum.  https://bitcointalk.org/index.php?board=160.0

As of right now Bitmain does not appear to have any of these for sale.  I recommend you wait until they announce a batch for sale, then purchase directly from them.
ASIC miners are expensive and you might as well get a warranty if you are going to spend that much money. 
Additionally, there are very few reputable re-sellers of ASIC mining equipment.  And most will not sell you a miner unless you pay them more than they can make by mining with it themselves.  That means you will pay more for the miner than you will be able to recover from mining.

There is no easy money here I guarantee.

Those L3+'s will produce more value per day than the S9's.  At least they have been since they came out.  One L3+ will produce the USD equivalent of about 3x S9's per day.  This ratio fluctuates somewhat everyday and that level of profitability may not exist forever.  But you should take advantage of it while it does.

So I would do what Mr. Phil suggested,  just above me.  Mine LTC or Digibyte or whatever is the most profitable Scrypt coin with those L3's.
Then convert whatever you are mining into btc on one of the exchanges.

Take a look at https://whattomine.com/asic to help you decide the best coins to mine.  Select only Scrypt.  Turn off SHA256 and the other algorithms.

Or - the most simple option is to point your miners to Nicehash.  They will mine the more profitable coins for you and then pay you in btc.  
Go to https://new.nicehash.com/asic-mining to figure out the closest servers to you.

I have 2x L3+'s and they mine on Nicehash almost exclusively.  Especially, when I don't have time to keep up with what scrypt coin is the most profitable every day.

Besides buying direct from the manufacturer, you can count on one hand the reputable Antminer re-sellers in the world.
It's not worth the risk or the inflated cost.

You are not going to get a current production miner for less than what Bitmain charges direct.  If that is what you are looking for.  There is no wholesale price for miners unless you are buying 500+ direct from Bitmain.

I have probably purchased 30-40 miners from them over the years.  You just have to have btc ready so when Bitmain starts selling you can jump on and purchase within the first hour.

I would not trust any secondary vendor, except Bitmain Warranty.  Nor do you want to pay their inflated prices on Amazon, eBay, Alibaba, etc.  At least not for an S9.  Bitmain's prices for the L3+ and D3 are much lower in relation to how long they will take to pay for themselves as compared to the S9.  So I can see paying more for them with a secondary vendor.

Build some GPU rigs in the interim while you are waiting for the next batch.

Well I agree, at least when I have a problem.  And I don't speak Mandarin!  But when I don't have issues I'm happy it's so hard to get a miner.  It's a huge barrier to entry for your average Joe.  At least in the US.  If there were new ones available all the time on Amazon the difficulty would be even higher than it is now.  Which I know seems unimaginable.  But after years of btc being a thing, it's still hard for most people to easily get their magic money machine.  And most of the time I'm fine with that.

Yes, Bitmain's name has to show on the transfer exactly as they instructed.  Transfers are not like personal checks, where anything close will work for depositing.
Yet another reason why it's worth the time to set up on an exchange and transact in btc.  Or ltc, or dash.  Anything but fiat.  It's a sure way to screw up your order.

Yeah.  Do the math.  It's a good price.  Bitmain could sell theirs for more.  The ROI on the L3+ is a lot better than the S9.

The article "Blockstream having patents in Segwit makes all the weird pieces of the last three years fall perfectly into place" may be an editorial and without merit.  But I would give Mr. Maxwell's rebuttal the same one star review.  He may be an "executive" of Blockstream (which given his Core duties is a conflict of interest), but that does not mean he knows whether patents have been applied for or not.  His response boils down to "trust me."  
And you see, it's that very loose element of the human condition and lack there of, that inspired Satoshi to invent bitcoin.
So, no.  I at least won't trust any of you.  History has shown that to be bad for my financial health.

I lump all righteously pro SegWit posters together because I can't discern their true motives.
The movement has lost all credibility in my view because of their censorship, scare tactics and strategic media campaigning.  All of their arguments conveniently leave out the part about how Blockstream will fail if it's not implemented.

It's this crap that makes all pro SegWit folks suspect in my mind.  I read this, but in my head I see:

"BTC urgently needs to activate SegWit as soon as possible for the sake of MY BLOCKSTREAM HOLDINGS!"