If the phone's radio supports it, you shouldn't need to enable - just insert a SIM for a network that uses it (as long as the phone is not network-locked), or roam onto that network. Check your mobile data settings to ensure that 3G data is on (i.e. mobile data active and the network mode not locked to 2G only).
According to the specs, all S3 models apart from the CDMA one should support this. If the phone still won't see the network after checking the settings, it might be best to post in the main forum where we can discuss things properly.