So far, what I have understood is:
* You have 3 RDP servers, with static IPs 192.168.0.1/2/3
* You have a Zentyal installation with a public interface 77.170.180.160, internal interface with what IP?
* You have some Switch (I guess) to which all servers and internal users (including yourself) are connected
* These users have Internet connection (i.e., Zentyal installations is working properly as a gateway)
* Users (internal and external) can connect to the RDP servers
* Ports 3389/tcp and 3389/udp are being properly forwarded on Zentyals's (although if you are only using port 3389, only 1 RDP server is accessibel from the outside, right?)
* You have your Zentyal's with a DHCP serving IPs in the range 192.168.0.100-200
Note that even if I don't manage to help you, at least everybody else will have a better understanding of your issues.
Your main problem is that inside a RDP session in one of the servers, users have no Internet access.
Questions:
- Have you configured each RDP server with all the information, gateway, DNS, etc?
If you have physical access to one of those machines, do you have Internet access?
- Can you ping some external server by IP (try "ping 8.8.8.8")?
- Can you ping some external server by name (try "ping google.com")?