1280x1024

I'd do one on the top and the other 2 splitting the bottom (extra width is a lot better than extra height...)

Main: 1280x512
Clones: 640x512

The numbers for maximizer are how far from the edge of each screen your window is... It's: top,bottom,left,right

For example, for Clone 1(bottom left) (512 from the top edge, 0 from the bottom edge, 0 from the left edge, and 640 from the right edge)

Full maximizer settings:
Main Maximizer: 0, 512, 0, 0
Clone1 maximizer: 512, 0, 0, 640
Clone2 Maximizer: 512, 0, 640, 0