what kind of consultancy?
if it was about bitcoin and/or cryptocurrency, wouldn't it be ironic you are asking this?
or your payment would be part of whatever inside that thing you need  to decrypt?

how could you say (know) you have 3 BTC but don't have a bitcoin address as reference?
you mentioned "Bitcoin Core" but then you were asked to do something related to a nano ledger s?
no consistency and none of his info make sense, he should start over telling the story from the beginning with simple words

I have a feeling this is another variation thread of "Please help decrypting my wallet and I will pay handsomely"

It is sounding like you have been lied to.

It is possible that the sender might have sent you some private keys, but from your explanation so far, it sounds more likely that the sender tricked you into downloading some fake software that pretends to be a bitcoin wallet and displays a fake amount of coins to make you think you've received bitcoins.

You would need a Ledger Nano S to actually use anything related to it; installing anything won't turn your computer into a hardware wallet. Where did you get the software? While your computer scan might not have any malware infections, some malwares are undetectable.

What were you given when you got the Bitcoins? You can't get Bitcoins without actually getting a file or a string of alphanumeric letters.

How are they stored in your hard drive?
How exactly did he send it to you?
Did you give him a bitcoin address?
Or is it stored in a wallet.dat file?

PS be very careful with the information you share.
Do not send any files to anyone that PM's you, and if you're sharing stuff you're not certain of then just share the first 3 or so characters.