I made up a cable which split each wire from the Digitech FS3X switch into three, then attached three TRS jacks to the other end. I had to bind up all the wires.
This worked, so I got Custom-Lynx.co.uk to make me on up to length, and presto, tap tempo going to each from the switch! It's not a standard type of cable, and I found Cleartone unwilling to make it. But Custom-Lynx listened carefully to what I was trying to do, and the resulting very smart bright red multicable works perfectly.