KyleTM

I know cases that can hold 2 motherboards but for 3 you may want to make something custom.

Which exact PSU you have in mind? Just because there is 460W on the packaging doesn't always mean it can handle 460W, there is a lot of sketchy marketing with many sketchy manufacturers in the PSU industry.

-Moved thread to cases and power supplies-

You would really think that this is a common sense but I also see a lot of people recommend PSUs based on efficiency. And I see them even more often to overestimate the needed wattage for the system by a lot.
Overkill very expensive PSU (1000W 80+ Platinum) for system that will draw 400W from the wall at most where 500W-550W 80+ lower rated PSU which costs half as much would actualy work with the same efficiency due to working in the most efficient 50% load.

I'm pretty sure you can run that system with two 980tis on 750W. I'd personally go 850 but I'm pretty sure it would run without hiccups.

I've had a cheap PSU go boom and take out a mobo and a 2x250GB HDD array in 2011. 
That machine was at least 7-8 years old at that point and was a home server so I didn't care at the timr