Isn't that 3 contributor limit just a virtual limit. Assume there are 5 contributors (A, B, C, D, E) for a project. The organization could do this:

1. Create two private copies of that repository

2. A, B, C contribute to the first repository

3. C, D, E contribute to the second repository.

4. C will maintain the synchronization

5. One could write scripts for performing synchronization.