I've got all three working correctly now; I have seen a slight slow down in the Mac's performance which I'm not happy about but then also not distraught. And three screens are blissful.
In case anyone's wondering how it's done:
1) Move the original video card to slot 4 and run your left and right monitors off it
2) Buy an ATI Radeon x1900 XT card off eBay for slot 1 and run your main monitor off that
Ridiculous how simple that sounds considering how long it took me to get to it. But there you go. Happy now.
Thanks again,
William