They are indeed in a canon threesome!

I actually do think we got hints that Simon likes Julia, they were just sort of gross hints - like, he gets really pissy about Julia's fast ways in a very specific fashion, and I think we are meant to take from that that Simon is attracted to Julia, and annoyed by the fact that he is attracted to Julia. Obviously this is not okay behavior (as a lot of Simon's behavior is not really okay behavior) so more exploration of that would definitely be a good thing . . .