Phone Info
From what I've seen, this is the only app that works for quickly getting into the menu required to change this on the S3. After installing and opening the app, select "Device information." Then scroll down to the first drop box. It should say LTE/CDMA/EvDo by default. Tap on it, and select "CDMA auto (PRL)." Hit the back button to exit out of the app in order to make sure it saved the setting change. This will put your phone into 3G-only mode.
Great info. Thank you. Could you give a quick rundown of what the several other choices in the pull-down menu are?
Won't work with Bionic on leaked ICS either.