Hi Daniel,
I would suggest putting the other two channels under a button and keeping them active, then the fog button will trigger the fog. Or is the machine already active with only those two channels?
If that is the case, you could split the machine into two fixtures, so you have the "Fog" channel as a fog channel, but also the intensity channel as a fog channel.
Let me know if I can help.
Greetings,
Bootsy