"Love triangle" is a very broad term that you could use to describe pretty much any romantic configuration/conflict of three people.>I'm kind of inclined to say it's ... more of a relationship with a 3rd wheel
Star is superficially the third wheel in the traditional sense, but she has never actually intentionally gotten in the way of them as a couple, up until her confession (right before she removed herself from their lives, "forever").
Marco chose to interrupt his date with Jackie in "Bon Bon" to check on her, the "date" at the concert was the three of them bonding as friends (Star is the one who bought the tickets, and she invited Jackie over Marco's objections) and Star chose to leave when Marco and Jackie started getting romantic, and "Starcrushed" is about Marco and Star trying to work through an awkward place in their friendship, at a party in their own shared house, with Jackie setting herself aside and encouraging them to work things out.
Beyond the surface apperance and traditional definition, Jackie is the true "third wheel" to Marco and Star's relationship. Out of the three of them, their friendship is the most deep and intimate bond by a wide margin. They would, and have, drop everything to help the other, even risking their lives to save each other.
Marco and Jackie are in a relationship, but they still don't really know each other. It's still just a freshman high-school romance, and now it seems like it's Too Little Too Late
for them to move beyond that.
And "tricycle" is completely inaccurate; there is no polyamory involved. Star and Jackie both like Marco, neither of them have interest in each other beyond casual friendship, and they're definitely not willing to share him.