Buy cheap AR-15's. It's the most common mechbox design. Lots of interchanging parts, in general. Worst case, maybe you spend 300-400 on three guns, one or two break, you assemble the working bits back into functional guns, perhaps with upgrades.
I would highly recommend grabbing 6.01mm aluminum barrels (madbull) for everything. The expensive stuff will be brass and generally be smoother inside, but the madbulls are very much worth it for their cheap prices (IIRC about 30-40 a gun).