Thank you Brazzy for answering,
Unfortunately, these settings won't let me hear any vocoding fx. The mike is plugged in the mic input, same channel as the guitar's (I haven't got any Variax to use that input), and it's not detected by the X3. So I hear a tiny guitar sound with no modulation, even if I push the mic's gain or scream like a cow.
I must add that microphone (shure beta57, dynamic) and XLR cable are okay (tested on other devices) and that when I use the X3 as a mic preamp, the XLR input of the X3 works well.
Did anyone manage to use that effect with an X3pro? Only one video is available online, showing an X3 played as a vocoder, but it's with a desktop model and doesn't show anything about settings or patching.
I wonder if my device is okay.. Just when I finaly found how to tweak a realistic guitar tone with it.