Obligatory disclaimer: This requires messing around with the windows command prompt. If you aren't comfortable with the fact this CAN render your computer unable to boot if you aren't very careful, don't do it! Also, I cannot be held liable for any damage you do while attempting this, blah, blah, all that rubbish. As long as you only stick to using the MKLINK command, you should be fine.
With all the talk about using SSD's for The Sims 3, I decided to do the same myself. The problem I have is, I have a lot of app's installed on my SSD, and it's only 120GB. That's big enough to store The Sims 3 and all it's expansions, but not my save games, store content, extra worlds, mods, etc. The reason for that is because The Sims 3 stores all of this content under "My Documents", and I just don't have enough room on the drive to move the ENTIRE folder.
Ladies and Gentlemen, I introduce you to the MKLINK command!
Using this is going to be a bit techy, and as far as I know, only works on Windows. You are going to need to use an Elevated Command Prompt (Windows Vista/7 users, click start, search for CMD, right-click the CMD result and select "Run as Administrator." Windows 8 users? Sorry, I have no idea how to do it in that, but try the same as Win7 and hope), which should look like this.
I'm going to use how I have my folders set up in this example, but you will need to change it depending on your own personal setup. The command you want will look similar to this.
Once you put that in, windows should say "Junction created for <text here>". If you go and check your Documents\Electronic Arts folder, you should now see this...
The folder is showing as a "Shortcut", which is exactly what we wanted! Whenever The Sims 3 tries to access this shortcut, it will find the files wherever you decided to put them (in my case, on my C: drive) rather than where it WANTS them to be (in the Documents folder on the D: drive, in my case).
I've tried to explain this with as little tech lingo as possible, but if anyone needs a better explanation or help, let me know.