You don't switch between iPad2 & iPad3 with the simulator. Your options with the simulator is to:
- change between non-retina and retina resolutions and
- iOS versions.
If you're debugging resolution issues, note on the Hardware menu, you can select between iPad and iPad retina.
You can also go to XCode preferences, Download and download older iOS simulators.
Outside of that, your options are to get your hands on actual devices which is the ultimate way to test what will really happen :)