You have to figure out how to make the RDP go out on a different port. That is the only way....
personally, for simplicity sake. I'd install VNC on all 3 computers. You could then RDP into 1 of them, and VNC from that one to the other 2. Will be easier, and adds another layer of security to your self, as even if someone did break the RDP, they'd then have to break the VNC.
NNNOOOOO!!!!!!!!!!!!!!! I've got it! hahaha....
setup your router for remote login. Put all of your PC's back to using the standard port for RDP. Now. what you'll do is
go to Test@mydomain.com(: / i forget which) 192.168.0.1 (or whatever the IP for your router is). You then can set your router to foward the default RDP port to whichever PC you need to get in. If you then later want to get into a different PC, you'd have to log back into the router and modify the port forewarding to do to a that PC..... lots of work tho