All the Afrihost Huawei E3531 USB Dongle is set by default to connect automatically. You will not be able to change the settings to connect only to 3G. If the device that you are using was not bought from us than you could try the below steps to see if the software provides a different setting.
Log into the web interface using the default gateway: 192.168.1.1
On the menu on the left, select Network Settings
In the screen that appears click on the drop down menu under the Network Type to select the network type you would like to connect to. By default it will be set to automatic.