There are no specific setting to enable 3G more than the network mode that you can find via Settings > More... > Mobile networks > Network mode, make sure that it's WCDMA (preferred)/GSM. Also make sure that Mobile data traffic is marked in mobile networks.
If you still do not have a 3G connection, make sure that your internet APN is correct via Settings > More... > Mobile networks > Access Point Names. You can get the right settings from your operator.
