I'll second the LVM idea. You can use samba to simply share the entire space of the 3 additional drives, but they won't be offered as one huge 750 Gb space, it will be offered as 3 seperate 250 Gb spaces. If somebody tried to upload a 300 Gb file, they couldn't.
You could use LVM to make the 3 drives feel like one single drive, which will give you about 700 Gb of usable space. I have 4 250 Gb satas in a hardware raid 5, and I have about 700 usable. The 250 is an estimate size, and not very accurate. it is more like 237 Gb each I think. In any case, this page has good suggestions for how to setup the LVM, and then simply mount the LVM somewhere, and share that location through samba.