I had a PC laptop and had The Sims 3 installed on it through Steam.
I figure that Mac and PC have different platforms, but is there any way to have the game installed on my Mac?
The Version of Sims 3 on Steam is Windows PC only.
However, you should be able to activate the Serial Numbers provided by Steam on Origin and then play it through Origin on your Mac. This requires to redownload it though.
You can download Origin here:
Make sure you click on Mac before you download and you need an EA Account.