First of all love triangles and threesomes are not the same thing, let's just get that one straight. A love triangle is when you have feelings for two people and one or both of them have feelings for you too. A threesome is when three people have sex with each other, I don't think I've read a single book where that one happens although I wouldn't be surprised if they were out there.
I'm going to list a few books:
Fangirl by Rainbow Rowell
Solving for Ex by Leigh Ann Kopans
Love Letters to the Dead by Ava Dellaira
Breath, Annie, Breath by Miranda Kenneally
Anna and the French Kiss by Stephanie Perkins
Say What you Will by Cammie McGovern
Monument 14 by Emmy Laybourne
Pointe by Brandy Colbert
I'm going to list a few more books:
Cinder by Marissa Meyer
Eleanor and Park by Rainbow Rowell
Divergent by Veronica Roth
No Parking at End Times by Bryan Bliss
The False Prince by Jennifer Nielsen
The Coldest Girl in Coldtown by Holly Black
Pushing the Limit by Katie McGarry
Between the Lines by Jodi Picoult
Now, I could go on with these list all day, or at least until I ran out of books on my Goodreads. The first list are books where kids have sex, either the main characters or supporting characters, the second list are books where they don't, and there are NO threesomes. There may be a love triangle or two, but no threesomes. On top of that, in the books where the characters are having sex, it's a big deal. There are characters in books who wont have sex because they aren't ready, characters in books who have sex because they are ready, characters in books who make bad sexual choices and have to lives with the consequences. While teens are reading these book they're thinking that they don't want to be the girl who has sex with a guy to get him to like her but then he leaves her. Their "friends" might pressure them to do just that, but after read "insert book here" they know what the possible negative outcomes could be. Katie McGarry's Pushing the Limit has a super sexy looking cover.
There is nothing wrong with having sex in teen books. Teens have been having sex since the beginning of time, they just don't try to hide it anymore. Also since the beginning of time, teens have been avoiding talking to their parents about sex. I would rather have my daughter read How to Love by Katie Cotugno where a teenager has been raising her child alone (well with her family) after her boyfriend who had a drug problem (that she thought she could change) cuts out of town. I'd want my teen daughter to take a closer look at her own boyfriend and think "is this kid worth is" and when it comes to sex I'd want her to think "am I ready for this possible consequence". That book can and may inspire those thoughts. If she's not going to talk to me, and she might not, I want her getting the right messages from somewhere. I'd rather her read a YA book than watch the Anaconda music video and give that dance move a try in the streets.
All this threesome business, I have no idea what that's all about. It hasn't appeared in any of my books so I don't even know. What I do know is that I'm sick and tired of hearing "YA books are like this", "YA books are like that", "YA books are inferior because blah, blah, blah." YA aka Young Adult is an age specification; within that age there are multiple genres. Stop your judging and generalizing. Unless you've read ever YA book in creation and have exact statistics for me, I don't want to hear it.
Here ends the rant.