Can I use Shimano trigger shifter with my X3 SRam gears? My bike has SRam X3 gears and a grip shifter. I'd like to change for a trigger shifter. In the bike shop where I bought it, they say that I can change for a Shimano shifter (they don't have SRam). I've read that they aren' compatible.

Is it true? How can I discover if they are compatible before changing the shifter and the grip?
SRAM X series and most (all?) of the road stuff has a 1:1 ratio. Shimano is (I believe) 1.5:1; you are asking for drive-train tuning hell if you mix.

Cassettes, mix all you want. Shifters and derailleurs should be matched.