TS3 does not work well on Macs
TS3 was made for PC and works best if you use a partition. But if you don't, the game is routed through another program and creates all kinds of glitches in game play. It's a memory hog as well and you need to shut down everything else to play. It lags and crashes.
Check out the Mac users reviews on TS3 website for more info.
If you're new to playing a Sims game, I'd try S2, which doesn't have the problems S3 does. That or wait and see how S4, coming out in a few months, plays out and how it's reviews go.