I recently decided to build a system using following parts:
AMD FX X8 8350 BOX
ASROCK 970 EXTREME4
3x PCI-E XFX AMD RADEON HD 7870 Black Edition 1050M
8 GB, CORSAIR Vengeance RAM
2x 5200rpm HDDs
1x generic SSD (not sure which one yet)
So I found a powers supply that I thought would fit my needs: CORSAIR CX 750W 80 PLUS Bronze
When I went to the first wattage calculator (the Asus one) it gave me a value of 1200 Watts. That seems far too much so I tried this wattage calculator that I've used in the past http://www.extreme.outervision.com/PSUEngine
It gave me a value of 589 Watts.
Do you this the Corsair power supply that I mentioned would be able to handle this system with 3 GPUs?
An 8350 and three 7870s? I'm not sure you can tri fire those cards, might want to double check that.
All of AMDs high end CPUs right now are 125W CPUs. The 7870 has a TDP of around 175W, so 3 would be 525W. I like to add in another 50W or so for board and other stuff and you have a total of 700W total system power. You're going to have some issues with this however as most 750W PSUs only have four 6pin PCIe plugs and you'll need 6 for the tri fire. As long as you get a quality PSU that can do 700W+ on the 12V rail(s) and has 6 PCIe 6pin plugs you'll be fine.
If I'm right and you can't tri fire and only do two then we knock 175 off that 700W figure and come up with 525W. The new 650W units with 4 plugs can do this, as can the 750W. If you want to OC that CPU to help out I would get a 750W unit.
Edit: Did some fast checking, you can only run two.
I'm not up to speed on tri cards. Normally only the top end of each generation can do more then two cards. So GTX670/680 probably can, while the 7950/70 can. Yes I know its funny that the 580 can while the faster 660TI can't, but the 580 was top for its generation while the 660TI isn't. (I'm guessing its not anyways...) Any reason why you want three?