I would just go with 7950 crossfire, tri-fire has multiple issues from what I hear. You can easily overclock the 7950's to 7970 crossfire performance and surpass it saving you $200. Stock versus stock the 7970 has only a 5-10 percent advantage depending on the game.
First of all, your motherboard is incompatible to 3570k because it uses an AM3+ socket which is only compatible to AMD CPU's. In order for 3570k to work, you need a LGA1155 socket mobo like Asrock Z77 Extreme 4. I'd go with Crossfire 7950's (Gigabyte Model) it's almost good as 7970's crossfire but cheaper bang for the buck.