I have the regular splinter cell that i just picked up...I've been playing it for some time now. but when I try to use the thermal googles key 3 my game shuts down!!! how do I fix this?

Have you patched the game to 1.2b?

A secondary possibility is that your computer is marginally able to play the game and using thermal goggles puts enough additional strain on your system to where it kicks you out of the game. This is most often seen on machines using an onboard graphics chip like those made by Intel. What do you have for a graphics card? If this the case for you there are only a couple of options. 1) don't use thermals or 2) get a supported video card.