Sims 3 Keeps Freezing/Crashing on my Mac!

There is nothing you can do to completely avoid this.  It's always going to happen unless you play in extremely short time intervals, i.e. less than 30 minutes.  There are things you can do to extend your playing time before the crash happens.  

I'm not sure how much troubleshooting you've already tried, but here's some advice:

If you actually want to "play" the game as it's supposed to work, then get a PC or Bootcamp & Windows for your Mac. The game is not natively written for our systems (I have a mid-2009 Macbook Pro). As long as EA uses Cider, a "wrapper" for the game that enables Macs to play Windows games, it's going to crash with memory issues.

Do not follow EA game advisors' advice without taking into account that many times their advice is wrong for Mac users and can harm our systems. Just use common sense when it comes to their "advice."

Here are two good posts from the Sims 3 Mac forum that have a couple of useful "band-aids" to get the game to work temporarily:
-- http://forum.thesims3.com/jforum/posts/list/658546.page
-- http://forum.thesims3.com/jforum/posts/list/611723.page

Best of luck in your battle to play the Sims 3.