There is no “love triangle” in Star Wars OR Empire. In ANH Han only says he’s interested in her to wind up Luke. Luke is the only one to show any interest in her but there is nowhere that Leia shows anything more than friendship towards Luke. The kiss on the cheek during the chasm sequence is nothing more than a kiss for luck. In ESB Leia only kisses Luke in the medical bay to wind Han up. The kiss she gives Luke after they rescue him is nothing more than a friendly kiss. She has already announced her love for Han by this point. Luke doesn’t show any romantic interest in her at all.
Yet when we get to ROTJ, Han suddenly thinks Luke is a rival and becomes this jealous idiot? All that has to go. Han became a pretty much useless character as it is in Jedi, but this jealousy crap put it over the edge.
I think you can make this argument because of the way it’s played but it’s not actually the case. Mostly bc the whole “farm boy rescues the princess” is inherently romantic. And Luke is clearly interested in her romantically; with Han it’s less definitive but the very nature of their contentiousness implies sexual tension - this is one of the oldest tricks in the book.
By the end of the first film things are up in the air but the princess has “bonded” in different ways with both men - basically the classic “sex” (Han) vs “marriage” (Luke) contrast. Though it’s not clear where it will go, generally speaking a moralistic story like Star Wars would reward Luke in this scenario.
I think with Empire they moved away from the Luke part of it bc of that chemistry between Ford and Fisher, which was the smart play.
I agree that by ROTJ it was way past decided but I don’t think it’s completely implausible Han thinks that Luke is in the picture. Mostly it speaks to the insecurity behind his bravado, which is a good bit of character that shouldn’t be lost.