This would add balance in two main ways:
1. Transporter no longer OP in the endgame just by existing. I don't have a problem with them being powerful, it's the fact that they simply have no limit on their ability. Like a Veteran, Jailor, Mayor, or any other powerful Town role, a transporter would have to time their ability right in order to be powerful.
2. Transporter would be easier to lie about and harder to prove.
I don't expect this to get a lot of support because, let's face it, Transporter is pretty fun as it is. You can cause nightly chaos with pretty much no restrictions. As an evil role, however, it's much easier to see how Transporter is imbalanced. It's made worse by the fact that, unlike other "Town Power" roles, there can be many of them at once. Giving them only 3 transportation would force them to be smarter about their ability and give evil roles a legitimate chance to claim transporter.