Start with a HD 5870 video card XFX HD-587X-ZNFC Radeon HD 5870 $400 for example.
And 3 LCD monitors Dell P2210 22" 1680x1050 $230 This example has a DP connector and also Portrait (pivot) mode so you can choose a 5040x1050 virtual display or a 3150x1680 virtual display.