You don't have to have a 3-button switch, specifically. You can still get a dual footswitch to control different functions in looper mode. Nick mentioned the section starts on UM p33 and the specific settings are at the top of p35. That's either the printed manual or the download PDF version in the documentation section of the TF page. All that being said, from what you've said, I think a 3-button footswitch would fit your needs better than your current 2-button. Just as long as the switches are momentary, not latched.
I'm using an Ernie Ball VP Jr. 25k, although I've used several others, including a dirt cheap plastic toy that I picked up new for $20 as an experiment 10 years ago. Most should work, though, and although I don't own them, I have seen others use both M-Audio and Moog volume pedals successfully.
Please note that they don't connect like most expression pedals. Instead, use a standard 1/4" mono guitar cable and connect to the instrument output on the volume pedal, not the input or a dedicated expression output. Eventide set it up this way because it works with almost any volume pedal, not just specialized expression pedals. Ideally, the volume pedal should be 25k, although I've seen 250k and 500k pedals work (the taper isn't as smooth though).
And you're welcome. I just hope that maybe some of this will help you get your TF working the way you want it to now, instead of having to talk ET into changing the way it works and then waiting for a software update.
