1. You can't FIT three 4870s at all. You can run three 30" Cinema Displays with two GT 120, for crying out loud.
2. The power supply is not the problem, the PCIe bus is.
3. You can't change the bus.
4. You would need an entirely different logic board because the power supply has nothing to do with connecting external power for graphics cards in a Mac, but the number of 6-pin molex connectors on the logic board does.
Build a Hackintosh if you need three for whatever incorrect reason.