Ofono is used for controlling the cellular modem and it can be configured via dbus. The relevant properties are accessible through the RadioSettings interface: http://git.kernel.org/cgit/network/o...ttings-api.txt. However "Prefer 3G" sets the TechnologyPreference to "umts", which leads to automatic 3G/2G. So forcing 3G doesn't seem to be directly possible. As a workaround, you could try to set the GsmBand to some band that is not used in your country.