Overbridge using the three Digiboxes

I’m trying to update my setup including the three Elektron digi boxes connected via USB to Overbridge on Ableton. When selecting the audio device on Ableton I choose my audio inteface as input and output although I’ve seen on some Youtube videos some users selecting any of the devices as input and output… Does anyone have any advice about it?
On the other hand, could anyone advice on a three digibox setup how do you manage the FX returns on Ableton.