If you're going to use Eyefinity to stretch your gaming screen across all three monitors, they must all have the same resolution and orientation. If you're just going to have a multi-monitor display with the game on one and various web browsers, spread sheets, etc open on the Windows desktop on the others, then your monitors can be different sizes and orientations. I run a 1200x1600 UltraSharp 2001FP next to my 2560x1600 UltraSharp 3007WFP.
Radeon HD7950 3GB goes for $300 Canadian.
i7-6700K, NT06-Pro, GA-Z170N-Gaming5, 32 GiB, RX Vega56, SM951, 5TB HDD, Blu-ray, FTZ01, SX800-LTi, C32HG70, RK-9000BR, MX518