Share your experience!
Hi and welcome to the community! Since you're new please be sure that you have checked out our Discussion guidelines.
The options in your phone doesn't look like they do in mine so i suspect your phone uses an operator specific software where the operator has removed this option. But the option "Enable 3G only" you listed should switch you to 3G.
That option should set your phone to only use WCDMA. Not having it enabled should make your phone use both WCDMA and GSM.
What happens if you select "Network operators" after enabling 3G only? Does it find any networks?
Are you sure that there is coverage for 3G in your area? From what you are describing the phone is set to only use 3G but it can't find any 3G-networks.
In that case i'm afraid it sounds like there's something wrong with your phone.
I suggest that you try to reinstall the phone software using Update Service..
If the issue should still remain I think that this needs to be examined and fixed at a repair center. For more information about how to submit your phone for repair and where, contact your local support team.