Do you think Justin sees Michael as a "threat" in his relationship with Brian?
No, I don’t think so. Justin’s a perceptive lil shit, and he pretty much has Michael’s number right from the second episode:
Justin: If he’s such a selfish prick, how come you’re always following him around?
Michael: I am not always following him around! We’re best friends. We except each other for who we are, no judgments, no obligations. No questions asked.
Justin: You’re just waiting for him to finish jerking you off.
Justin: You never got off. That’s why you’re still hanging around after all these years.
I mean, sure, it’s a bit crude, and not the most nuanced argument, but I think it does reveal that Justin has a pretty good idea right from the beginning what’s going on: Michael has feelings for Brian, and Brian doesn’t return them. Michael keeps telling Justin all these things about how Brian doesn’t do boyfriends and not to expect him to keep coming around or be interested in him past that initial fuck, and then……….Brian keeps coming around and despite all his bluster, doesn’t seem particularly keen to shake Justin off.
So I think Justin’s just pretty clear from the very beginning about all the dynamics at play here. He can tell that Brian isn’t interested in Michael romantically, and he can also pick up on that the way Brian treats him is unprecedented, since he’s going completely against everything even his best friend predicted would happen. I’d say that would actually make Justin feel pretty secure about his place in the equation. Now, of course Justin has other fears and insecurities about Brian and his relationship with him, but I really don’t think Michael was ever a concern to him, since he could tell from the very beginning that the way Brian views him is different from the way he views Michael.