It means Motocross Grand Prix. MX stands for Motocross as "X" can also be referred to as "cross" in some countries. I haven't personally played MXGP 3 yet but I've played MXGP 2 and it's worth playing I might say. The physics were alright, but you can download mods to further enhance your experience.
This person is right. I've played both MXGP 2 & 3 and so far, both games are cool and amazing to play. I also downloaded some mods to improve my gaming experience and I was not disappointed at all.