What case should I get if I need to run three PSUs?
I think you'd most likely have to mod a case, as there seem to only be a very small number that support two PSUs.
http://coolermaster-usa.com/microsite/hafstacker/
CaseLabs cases have 2 or 4 as standart.You can buy pedestals and increase the number of psu mounts.
Lian-Li does some big cases, I don't think any of them hold 3 PSUs as standard but a few have more than enough for 3 or possibly 4 if you can mount them. they even have conveniently places vents. Talking double wide cases here
much case, such expensive.