OK, I just tried setting up the three screens again, and everything seemed hunky-dory in the Piper Turbo arrow 3. Main monitor running in 4k 3840 side screens in 1920, settings mostly on high with my RTX 3060ti, Super DLSS on, and I was getting 36fps generally.
I don’t know what was going on last time, but whatever it was, it seems to be resolved now.