1. Yes, but I haven't seen any reviews of what kind of performance this would bring. Or even if it would yield a gain. Proof that it at least works in principle is that new EVGA card that has a GTS250 and a GTX275 on one board, the former to do PhysX.
2. It'll be done in software. Connect everything up like it's normal 3-card SLI and it should work. Maybe there's an option in the driver control panel?
3. Going by EVGA's lead, a GTS250?