I'm currently on a(nother) rewatch via Disney+, it can't hurt to keep viewership figures up. MacFarlane's Instagram bio (Imgsed mirror link) still just reads ...
“The Orville: New Horizons” now streaming on
@hulu
and@disneyplus!
... and his account remains active (last post was today), so it seems like that's about the best we can do besides bothering other people to watch it as well.
Hulu obviously spent big getting season 3 made. If they had gotten the response they were looking for, the show would already be renewed by now, so making sure the show has a long tail (lots of new people keep discovering and watching it) is probably our best bet.