Why would you want it to be 3G only? If you ever have no 3G signal then your phone will not be able to recieve any calls/texts.
Unless I’m mistaken 3g is also known as WCDMA. If you type *#*#INFO#*#* and click onto “Phone Information”. Scroll down and you’ll see a dropbox which says “WCDMA prefererred”. Change that to “WCDMA only” and there you go. Your phone is now 3g only.
I prefer “WCDMA preffered” that way the phone will stay on 3G unless there is no 3G signal. Then it will switch to 2G until a 3G signal can be found.