Newer generation Fractal Audio devices (e.g., Axe-Fx III, FM3 and FM9) do not use fixed MIDI CCs. You will have to assign a CC for the Metronome. This can be done via SETUP / MIDI on the unit or its editor. Assign a CC that is lower than #34 that isn't currently in use, sending a value of 64 or higher to turn the metronome on, and a value of 0 for off.
A little more detail:
Because the AxeFX III doesn't use fixed CCs, you have to assign them manually. The clever people at RJM created a system that translates certain CC commands to Fractal Audio's sysex format. So in the RJM editor, you'll see a large number of functions as pre-programmed commands. This is super handy, and it saves a lot of configuration effort. Keep in mind that the RJM system will convert any CC messages higher than #33 to sysex; any CC messages from #33 on down are sent as normal CC messages. Hence the recommendation above to use CC26.