It’s most obvious in ANH with Luke and Leia’s reactions. In fact I think within the context of that film, Luke and Leia are meant to get together. Han acts more like a mature big brother who plays “cool” with Leia.

Best evidence here is that scene after escaping the Death Star where Luke asked Han about Leia, and Han says no chance, this is clearly aimed at a bit of love triangle. A naive, caring guy vs your typical detached, cool guy. Clearly setting up.

But something must of happened between ANH and ESB, because Empire doesn’t really seem interested in playing up a love triangle. Leia kisses Luke in that instance to make Han jealous. The entire B plot of that movie is dedicated to their relationship, so it’d be weird for Luke to just show up again. But in the same token they’ve already set up a love triangle.

Had ROTJ stuck to its guns with the original script, either Han’s death or Luke walking away to focus on the Jedi would have made a far stronger conclusion to the triangle.