The Weak Attempt
As the title suggests, this kind of love triangle is a weak attempt. This is when we're fairly certain that the couple is a couple, when suddenly, another cute character comes in. But in this scenario, we know that there's really no way that the ship could sink. The weak attempt of a secondary love interest can come in any point of the story after the couple is most likely a couple. It's not the worst kind of love triangle but it still makes me a tiny bit anxious that the secondary love interest will steal our MC away. Very unlikely, but it's still a possibility.
Love at First Sight
I've noticed that a lot of times where books have love triangles in them that the MC ends up with the first character introduced in the novel. However, this isn't always the case, but there are some dropping hints about how the MC feels more towards character X. One of the more predictable type love triangles.
The Even Larger Act
We think they're established after the big act. They've confessed their love for each other *cough cough*, and we're relieved that the couple will be together for the remainder of the series and that we'll read cute couple scenes from now on. Then, the secondary love interest comes in after they're established. Our MC starts having second thoughts about which character is better and which one they should choose (Why would they even have second thoughts in the first place after the act?? But then again, this is a rare occurrence in YA books so whoever invented this is bound to have some other rare occurrences follow up, such as having the secondary love interest win...). I haven't come across this type of love triangle very much (SJM...) but when I do, it's hell, especially when the other books in the series haven't come out or the series just started.
Opinion On Love Triangles?
Love triangles are definitely not my thing. With our MC taking their time choosing between which character to be with, it drastically takes away time that they could have already been a couple. Though they add so much suspense/tension to reading the novel in addition to the plot, sometimes there's just too much going on. This isn't a bad tactic for authors to keep their readers hooked, but I'm one for more action, more mystery, more suspense, and less time spent with our indecisive main character. This might be a little selfish on my part, but sometimes the love interest who lost is an amazing person, and I don't want him/her to end up with someone other than our MC. The absolute worst is when OUR SHIP LOSES. Or sometimes we saw it coming but the ship we chose was such a better choice...