I have 3 Vapor X R9 280x running well on a Lepa 1000w Gold PSU. 1000w should be enough as long as the PSU is gold rated and can deliver at least 900w on peak power output. Also you need to consider that you will not be able to overclock your cards much from stock as then you simply wont have enough power.
Also another point is to ensure your PSU has at least 6 of the 8pin cables to power the video cards as you need 2 per card.
I had a Silverstone 1200W G Strider GOLD PSU that had only 4 8pin cables, so i ran the other 2 via the molex adapter supllied with the card, however the wires on the molex turned out to be too weak and the modular connections on the PSU have melted and made these 2 connectors unusable.
After that i got a LEPA 1000w Gold PSU which has 6 8pin connectors and was only 139 euro!!
That if PSU will handle does not depend on the fact if PSU is gold/bronze/platinum etc. .
If You have good 1000W psu without 80+ (let's say its 60% effective) it will give max 1000W but at the same time it will consume 1400W at peak (1000W+1000W*(100-60)%)
If You have good 1000W gold 80+ psu (let's say its min 90% effective) then it will also give max 1000W but will consume only 1100W at peak.
So... if PSU is 1000W and it's not 80+ it still can deliver 1000W (if it's good brand) it will simply consume more power. The extra power (eg. 400W) is lost for example by heating up PSU.