It's possible. A lot depends on how your clients connect through WinGate. The easiest option is to ensure that you have the traffic going through the HTTP Proxy. Then, restrict by policy.
This topic gives you a reasonable overview of how policies work.
What you want to do is (Basic, you can tune as you go along):
1. Add a recipient for "Everyone".
2. Set that Default (System) Rights are "Ignored".
3. In the recipient's advanced properties create three filters, each with one criterion underneath it. The criterion should be:
"This right is granted if HTTP URL contains domain1"
"This right is granted if HTTP URL contains domain2"
"This right is granted if HTTP URL contains domain3"
4. Then ok out and all should be sweet.
This only applies if you're using the proxy or intercepting WGIC or NAT through the proxy, though. If you are not using the proxy, let me know how you connect and then we'll take it from there.