To change the mode of the footswitch press Menu, go to utility menu, press menu, then go to the "footswitch" parameter:
Set it to PEDAL to get a stompbox mode
Set it to PRESET to set it in preset mode
In PEDAL mode, the 1/2/3 buttons will directly control REVERB/COMP/DRIVE on/off
In PRESET mode they will respectively call preset 1,2 and 3 in the selected bank
Please note: with the additional G-Switch, you will have access to both options at the same time.