Samsung Galaxy Grand Duos (I9082) Supports two SIM cards, and you can choose which SIM should be used as a data service mobile. To use 3G internet on your phone do as follows:
1. Open "Setting" App
2. Go to "SIM Card Manager"
3. Tap on "Data Service Network"
4. Choose the SIM as the Data SIM
Note that, only the first SIM slot supports 3G interent. So if you want to use 3G you must set your 3G SIM on the first SIM slot