you can access the selection of network as below:
System Settings-->tab "All settings"-->on WIRELESS & NETWORKS select "More"-->"Mobile networks"-->"Network Mode"-->click the sim card and a windor will prompt up for selection of 1. GSM/WCDMA (auto mode) 2. WCDMA only 3. GSM only. If you want to stay on 3G then you'll need to chose WCDMA only, but you'll face no connection/no signal if the place/location had no 3g signal cause it wont auto switch to Edge.