Pretty sure the Mark III uses the same footswitching as the previous marks before it. One pedal for Rhythm.Lead, one for Eq/Reverb, one for RHy1 to Rh2. The FU-3 is the large 6 button footswitch that attaches with 1 multipin connection on the Mark IV.
However even the FU-3 doesnt have switching capability for ClassA/Simul or reverb. It just switches 3 channels, Fx Loop, and Lead/Rhythm EQ.
well seeing as how i always have reverb on, and i dont mind switching the amp manually cause thats what im used to , the mk 3 footswitch should do just fine if it even works
I made a simple two channel switch pedal. I took an "insert" cable, stereo 1/4" -> 2 mono 1/4" and I plug the two mono ends into the channels I want to switch between (this pedal will only switch between two channels) in the back of the mark iv. I then found an enclosure, a dpdt (I actually used a 3pdt switch I had lying around, but only a dpdt is needed) stomp box switch, and a stereo 1/4" jack. Put the jack on the side of the box, the switch on top. I wired up the switch so that only one of the mono connections is shorted at any one time, thus allowing me to switch between two channels.
Its simple and works great. I also left room on the enclosure to add another jack and switch in the event I wanted to have an eq, fx loop, or class-a/simul-class switch. The pics give a better description and show the wiring layout.