No clue… The old firmware should pair too as far as I know (not an expert). The latest firmware on v1 makes it pretty much at par at v2. I only have one which is a v2.
One other thing to do is attempt to exclude (remove) the 3rd one. It may have been partially included. When that happens, you can’t include it anymore because it thinks it already is. Weird, but sometimes if you attempt to remove it that will surprisingly be successful. Then, you’re good to go around the loop again…
So I’m not exactly sure what I did that fixed it, but I clicked the (-) button and connect button on ST and it blinked red for awhile. Then I pressed the connect button on ST again and the (+) and join buttons at the same time on the remote and it found it. I basically was just trying different buttons combos and it worked