Access to more than one internal server with the same serivice

Do not create a NAT Rule for RDP. https://www.sophos.com/en-us/medialibrary/PDFs/technical-papers/sophos-rdp-exposed-the-threats-thats-already-at-your-door-wp.pdf

Let's assume that, another service, may xxxx port.

Its not possible. Any firewall product cannot figure out, where to route. The static principles are applied. You have Source, Destination and service to figure out, where to route. 

see, I have static IP (No IP) that fixed in my ISP Router, I opened a port in that router as following: wan=6666, lan=3333, ip=192.168.1.2 (WAN port of sophos).

the router connect to sophos WAN port, and I created DNat rule for 3333 port, all thing work ok, but I have 3 PCs use the same port 3333, I want to open ports in router as 7777, 8888, and every one go to the specific PC with port 3333.

I wish that be clear.

BR

Sami

Thats possible. Simply change the port as you like. Network devices can filter based on source, destination and port to apply a NAT rule. If this is unique, it will be applied to the matching traffic. 

Thats possible. Simply change the port as you like. Network devices can filter based on source, destination and port to apply a NAT rule. If this is unique, it will be applied to the matching traffic.