Thinking back on all the love-triangles across comic books, there are almost none that become truly memorable. Off the top of my head I can only think of Mary-Jane/Black Cat/Spider-Man, Cyclops/Jean Grey/Wolverine and Cyclops/Emma Frost/Jean Gray. And two of those have two of the three same participants. Even now we have a very prominent triangle in X-Force between Fantomex, Psylocke and Archangel (people just LOVE to get into complicated situations with psychic mutants), but do these triangles serve to heighten tension, or are they just a stand-in for real character development?== TEASER ==
In my opinion, and this is of course subject to who's writing what, they generally work as intended and can actually be a great way to add suspense and an air of unpredictability to certain situations. Recall all the way back at the beginning of Ultimate X-Men, for instance: Cyclops and Wolverine were, much like their 616 counterparts, vying for the attention of Jean Grey. Professor X, as an exercise to try and make them get over their differences, sends them on a dangerous assignment in the isolated Savage Land.
When Cyclops nearly falls off a cliff and Wolverine catches him, he remarks that the Professor's idea seems to have worked...just as Wolverine lets go and leaves him for dead before immediately leaping into bed with Jean when he returns. This was a shocking turn, but also didn't seem out of character for the ruthless assassin-turned-teaching assistant. And of course, the revelation that Cyclops survived only served to heighten the tension of him returning to the Xavier Institute.
Returning to a modern-day storyline in Uncanny X-Force, when you factor in Archangel's already fractured, unstable mindset when he transforms AND Fantomex's dubious morality (as well his suave, charming allure), you have so many possibilities for how this can go wrong that you could make entire arcs out of just that. Though you PROBABLY shouldn't, and I'll tell you why.
Problems arise with these kinds of stories when they are used merely for cheap drama or become the focal point of a book. Anyone still read Archie? The crux of that book is which girl will he choose, as well as his wacky high school antics, but since that question STILL hasn't been answered, most people aren't terribly interested in jumping onto the title with the fact in mind that it hasn't fundamentally changed in decades. However, the best (or worst) example I can think of is in New X-Men, which I understand was/is a book about teenagers teens have drama, both in real life and in comic book form, but it'd be almost impossible to diagram everything that was happening on that team. You'd be better off saying that half the team was in love with the other half, but only if the object of their affection has someone else vying for it. You'd think with such an even boy/girl ratio that everyone would find someone, but no, we're subjected to an incredible amount of teen angst, and that's at the heart of the love triangle problem, which both those books illustrate, the triangle becomes the focus of the story rather than being a device to add drama or excitement to it. It becomes the main course rather than the spice that adds flavor, and no one wants to eat an entire dish of oregano, no matter how delicious it might be in pasta.
Eventually, though, Jean chose Scott (and then Scott chose Emma, but that's another triangle...) and that drama came to an end, which is a part of why it remains such a classic: it ended. We, as readers, got closure, the characters got some development and long-lasting tension that's still referred back to in modern stories, and that was enough.
But what about those triangles that never really got going and never quite ended? I'm thinking mostly about Sue Storm/Reed Richards/Namor, or even that Spider-Man/Mary-Jane/Black Cat one I mentioned earlier. These actually, and perhaps unexpectedly, work out the best of all because they're constant and fun. There's very little chance that Sue would ever leave Reed for Namor, or that Black Cat will be a real threat to Mary-Jane (even post-Mephisto retcon all they've done is taken a couple of tumbles between the sheets), but these flirtatious challengers popping up adds a dynamic to the characters that is often lacking. It's easy to see Sue Storm as settled into her role, outside of being a superhero, as a wife and mother, but when this buff, brazen, arrogant king shows up and begins demanding her hand it marriage, it's a lot of cheeky fun to see her actually return the sideways glance he's giving, and you as a reader realize she's still got very human urges. One tends to root for Reed more because he's the better man and because they said their vows to each other, but it's interesting to see "Mrs. Richards" have more than one suitor, even if you know how it's going to end. Likewise Peter Parker, nerd extraordinaire, because of his wit, intelligence and cool-factor as Spider-Man, he has his own veritable love octagon, and the women love him for different reasons. That's enough to keep Pete guessing since the women generally fall into two camps of what they represent for him: one is being a responsible, timely man, and the other is being a wild, unpredictable kid. Which will he choose? Well, we still don't know for sure, but isn't it still fun to guess?