Here are the steps:
- Launch VZ Access Manager but don't connect
- Press Ctrl-D to launch the "hidden" system settings window
- The password is diagvzw
- Select "Settings" from the menu
- In the "Preferred operating mode" section, change it from Global to "Auto CDMA"
- Close out of VZ Access Manager and relaunch it
Now you will always connect only to the 3G network. You'll need to repeat these steps and go back to "Global" if you want to re-enable 4G mode.
And it's usually a good idea to always check for updates within VZ Access Manager - it will update the VZ software, Windows drivers, device firmware, etc.