C3000-100NAS remote access

> [...] They all three have static IP addresses. [...]

   LAN IP addresses?  Do you mean actual static addresses (configured on
the computers themselves), or reserved dynamic addresses (configured on
(the DHCP server on) the C3000 router section)?

> [...] I can forward Port 3389 to one of them [...]

   True, but you can forward other ports to the other two systems (or,
as I'd advise, to all three).

> [...] I use Remote Desktop App [...]

   On what, another Windows system, or a Mac, or some other?

> [...] and apparently are restritcted to that particular port.

   Not so.  You can specify a different port number in the "PC name"
field where your connection is defined.

   You might, for example, specify port-forwarding rules like the
following (all TCP):

             Ports
      External   Internal   Server Address

        14389       3389      PC1_LAN_addr
        14390       3389      PC2_LAN_addr
        14391       3389      PC3_LAN_addr

   On the client side, you'd specify a "PC name" like:

      <your_public_IP_address>:14389
      <your_public_IP_address>:14390
      <your_public_IP_address>:14391

   When exposing a well-known service like Remote Desktop to the outside
world, I would not use the default port (externally).  Why make it
easier for a lazy attacker to guess the right port?

   I believe that it's also possible to configure the Remote Desktop
server on each system to use a non-default port, but I'd prefer to keep
things simple for intra-LAN communication, and let the router translate
the odd-ball external port numbers for external traffic.