Well the problem with this is with the default port the RDC uses for the connections.
Try this:
The default port for RDC is 3389.
Assign computer #1 port 3389, computer #2 3390 and computer #3 3391
Here is the MS article to do this.
http://support.microsoft.com/default.aspx?scid=kb;en-us;306759
or:
1. Start Registry Editor.
2. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber
3. On the Edit menu, click Modify, and then click Decimal.
4. Type the new port number, and then click OK.
5. Quit Registry Editor.
The connection portion is the part where it would get hard.
When you connect to the non default pc, try using the ip address and :3390 or :3391
so open the RDC window and type the ip and such like this.. 123.456.7.1:3390
If this doesnt work, let me know, I might have another solution.