Usually it's the girl in the middle.
And the biggest problem is getting it convincing that she actually is torn between the two of them while putting in enough foreshadowing so that we are not surprised by the result. Alas, there are two schools of readers. One wants the triangle to be in actual doubt, so that we have conflict inherent in it. The other wants it to be clear which one they should root for, with the other being a contrast. This is a conflict for the writer. To be sure, even if you go for the second school you can make it too ridiculous that she's attracted to the wrong guy.
The classic Gothic trope: the woman at the mysterious house with mysterious happenings, and the two men, one of whom would end up behind the happenings and one of whom she would marry.
One panelist spoke of how a true triangle needs three sides so you need the relationship between the rivals. Some of these exist like when two brothers are in love with the same woman, but I think she overstated it. (I thought, but didn't have a chance to speak of, a Lord Darcy where you have a love triangle between the lord's son, his love, and the woman his father chose for him to marry, who never even appears. The father is murdered so his permission is not needed, but the woman's brother and father did it, and the son realized that she instigated it, and so quite properly marries the woman his father had quite properly chosen.)
I brought up the only semi-romantic triangle. In Teresa Frohort's Miserere, there's a triangle with a man, his lover, and his immensely clingy and jealous sister. And in one Doctor Who episode we have the sage, prudent, and cautious leader, and the younger hot-headed and angry one, and, of course, the daughter of the first who loves the second, too. Which means she stands for the society torn between two leaders, too. And we went off with that, and a panelist talked about incorpereal rivals, like a scientist's dedication to his science.
I also cited Girl Genius as a prime example, and others agreed. We have the main Agatha-Gil-Tarvek, and then we have Moloch, who's in love with Sanaa (and we have no evidence that she knows he's alive), and then there are Violetta and Snaug, and he appears to be clueless about the entire fact that they are after him. Come to think of it, there was also the Agatha-Gil-Zola triangle where she couldn't get in a word edgewise because Zola always needed to be rescued -- that one was fixed but good.
King Arthur came up, of course. Much discussion of that triangle. I pointed out that neither Guinevere nor Isolde had children in Arthurian legend to avoid the messy aspects. One audience member had heard of a story where Arthur and Guinevere had a son, whom Sir Kay ended up killing, but it may have been before they imported courtly love into the Arthurian mythos so that no one had reason to question the paternity of the child. Somewhat later, another audience member riffed on how if a woman is a noble or royalty, the question of succession makes it important that she chose and keep to her choice.
I didn't like the Hunger Games one because I never thought Katniss was strongly attracted to either one until the end. Another audience member really liked it because the characters weren't evil and the triangle sprung from and was resolved by character growth.