Unity Launcher Plus is a tool to make it easier to use multiple versions of Unity with multiple projects. After you've added them, it's just selecting what you want. I've personally tested it with only Unity 4 and newer, but should work going backwards as well.
Here's a video explaining how it works: https://www.youtube.com/watch?v=hWts9OWQHYc
**NOTE: This is a free, ad supported version of the software. There is another version that has ads removed if you want to directly support this project.