You can connect a TC Helicon Switch-3 or Switch-6 to the Footswitch jack of your VoiceLive 3 or VoiceLive 3 Extreme. You can also connect a second expression pedal in the Footswitch jack. 

Please note that the Footswitch jack is global and its assignment will apply across all presets. This means that any connected Switch or expression pedal assignment will be global. 

If you are using the Footswitch jack for a second expression pedal, make sure that you are using the same brand and model pedal for the best results.