There are so many ways this could be a problem, its difficult to say without knowing more information.You really need to read the front page of this group as to the sort of information that makes solving these problems easier.
e.g
Operating system
Version of Operating System
Version of Cordova
Plugins
Code samples
What have you tried?
1. Are you connecting to a DNS name or an IP address? if a DNS name is the DNS name registered on the external 3G network?
2. Do you have a timeout issues that you don't see on the local network as its faster?
3. Are you really making a 3G connection?
4. Does your network provider block certain connections?
5. What port are you connecting to?
6. Can you see the port and URL from the browser on the phone?
7. What does the error say in Safari debug or Chome? We don't even know which OS you are using!
Rob