Specs fro the P8Z77V from from the Asus website
2 x PCIe 3.0/2.0 x16 (x16 or dual x8)
1 x PCIe 2.0 x16 (x4 mode, black)
2 x PCIe 2.0 x1
2 x PCI
While the 'black' slot is physically x16, it only has x4 lanes, meaning a GPU in that slot will be restricted, if not starved, getting its data to/from the CPU.
The 'blue' and 'white' slots share x16 lanes, so if one were to put two GPUs in tho slots, they would each get x8 lanes, which will be reduced compared to a different motherboard that provides X16 to both slots, such as one from the P9X79 series.
As Juan Salvo pointed out in another thread, x8 might be satisfactory, but it's certainly not going to maximize the potential of whatever GPUs you're using.
Why skimp on the motherboard, especially if you can afford 3 GPUs?