I've got a used Roland FC-300 coming next week to use with my VG-99. I will give it a test with the Axe-FX 3 and report back.
I have used my GT-1000 to test MIDI functionality with the Axe-FX 3 and it seems to be fine with basic stuff - program and control changes.
Note that for anything to actually reflect the state of the Axe-FX 3 correctly, the foot controller would have to have explicit support for the Axe-FX 3 via the MIDI spec that
@FractalAudio has published. I know some vendors are working on it, but I'm not sure who has something working.