If you are planning to use the 275 for physX, then yes. SLI-NO.
Even if you are planning to use it for physx, it would be a waste of money. Get a 9800GT instead and you wont notice a differnece in performance as far as physX is concerned.
SLI is fussier than Crossfire about which cards can be combined: The closer to identical the fewer problems you are likely to encounter.
Do n't bother with a PhysX card: Dual GTX275s' have more than enough power to run rendering and PhysX without the complication of adding a third card.
I am not even sure it would be possible to triSLI the GTX 295, I thought SLI and CF are limited to 4 GPUs'?