@chorley20 

Hi,

There can only be one forwarding rule for any given port number. Otherwise the router wouldn't know to which device inside its local network (the three NAS drives in your case) it should send the data packet(s) to.

Usually this problem is solved by using different external port numbers, like shown in the example below.

Service Name    Device IP Address    External Port    Internal Port    Protocol
Plex_18             192.168.1.18             32402               32400             All
Plex_17             192.168.1.17             32401               32400             All
Plex_16             192.168.1.16             32400               32400             All

However, this of course means that the client software on the devices that access the three Plex servers from the Internet needs to be set to the correct port number for each Plex server. Whether that is possible is another question.