This should work fine over 3G if the MMS SS is externally-accessible. Given an MMS SS that has an external IP address and DNS entry, with an SSL certificate signed by an external authority (e.g. VeriSign, GoDaddy), with the appropriate ports open (443), it will work.
If it does not work on 3G but works on WiFi, you likely have something that's only accessible internally -- e.g. internal IP address, failure to have valid DNS entry externally, a self-signed SSL cert, blocked ports, etc.