Just to confirm, what Chino described is very much the optimal configuration. It's the only way you can get 16x lanes for both your cards and maximum performance from your Intel 750 (my setup is very similar to yours, just 980Ti's instead of Titans). If you put your Intel 750 in between the two Titans, it'll work, and you may not even realize anything is wrong, but because the slot is not Gen 3.0, the Intel 750's performance will be cut by almost half.
Sadly, it means that your bottom video card and your Intel 750 will be virtually touching. Once that 2nd GPU starts to droop a bit, it can even make a fan touch the backplate of the Intel 750, as recently happened to me. I've actually had to put a dime between the bottom GPU and my Intel 750 to separate them just far enough so that my middle fan on my 980Ti spins properly. Did that last night. Seems to be working okay, and did resolve the problem for now, actually lowered my fan noise a bit too.
As for EZ-plug, I just found out about it myself. There are actually two plugs on your board you should hook up, which deliver additional power to your motherboard for stability. One uses a CPU cable, 4 pin (though you'll probably have to use a split-8 cable, as I did), that plugs in right at the top of the board above and a little to the right of the CPU socket. The other, and this one is probably more important to your setup, is a plain old molex connector at the very very bottom of the board, just below where your Intel 750 will be plugged in in fact. The molex pins are parallel to the surface of the board, facing down - basically you have to look at the board edge-on to see them as they are covered by a shield. Your PSU almost certainly came with the proper cable for it. Plug it into any Peripheral plug on your PSU on one side and a female molex connector on the other into the board.
