This is only the way, how to force 3G ONLY on Lumia 620
1. Download Vietnam Rom
http://www.lumiafirmware.com/ser/RM-846/guid/059R2C9
2. Download Nokia Care Suite and Install It
http://www.mediafire.com/file/dmv50rav7byhkqk/Nokia+Care+Suite+5.5.132.1446_Full.exe
3. Put the Vietnam rom in one folder RM-846 on this location
C:\ProgramData\Nokia\Packages\Products\
4. Open Nokia Care and choose Product and support tool for store
5. Select or type "RM-846"
6. Klik program and recovery, then nokia care verify the rom
7. Klik start, and connect phone with power of mode
8. then Nokia Care flashing now
9. after complet boot, press ##3282 + Call
10 Field test app appear, klik acept, klik wcdma, on bottom right, klik setting, choose network type "3G"
then you can lock the 3g signal congrat.
Do With Your Own Risk :)