You can do it in 2 ways. Goto Settings - Phone - Band and select your network type to GSM or you can add 3G switch button to Comm Manager by editing the Comm Manager registry setting:
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\5]
@="DataDisconnection"
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\4]
@="AUTD"
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\3]
@="WLAN"
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\2]
@="Bluetooth"
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\1]
@="Phone"
[HKEY_LOCAL_MACHINE\Software\HTC\CommManager\0]
@="FlightMode"
Change the default value on one of these keys to 3G. Example, to change the Microsoft Direct Push Button to 3G, change "AUTD" to "3G". That's it, reboot device to apply the changes.
Here's the list of functions that supported by latest comm manager (Notes: some functions may not supported by your device): Phone, Bluetooth, WLAN, DataDisconnection, Vibrate, ActiveSync, AUTD, Ir, FlightMode, VoIP, InternetSharing, 3G, WiMAX.
