Remote desktop to multiple windows machines on a LAN with dynamic IP

"Define multiple port forwardings on your router. E.g. If you have three machines (a,b,c), you forward it like this:

router:33901 -> a:3390
router:33902 -> b:3390
router:33903 -> c:3390

When you want to connect (let's say to a), just use

external.dyndnsdotorg:33901

as rdp address."