I just forgot that you have to hold it for a second to get it to go into tap mode.
So I did get it working enough to suit my needs.
Glad it is fine for you. My explanation (the tough part) was in the case you wanted to use the DMC3 for everything, including setting the tap tempo from there.
Can I choose which parameters I control with the expression pedal?
Yes. This is explained in the online manual for the H9 control software:
The relevant section is under Presets screen -> Programming the expression pedal patch