I'll only share this solution.

Forward one port each to each machine such as...

8001 to machine 1
8002 to machine 2
8003 to machine 3

Now install TIGHTVNC's server on each machine and configure 8001 to machine 1, 8002 to machine 2, 8003 to machine 3. From the outside world, run the TightVNC client and select which port to get to each machine.

Bingo done.

A similar port forwarding might be done with Remote Desktop but given the security pitfalls of RD, I will not write how to do that.