This isn't considered a teen novel but I see no reason an intelligent, thoughtful teen couldn't appreciate it, especially someone already reading adult literature. The three in the title are girls, perhaps true friends who grew up together and almost made it to graduation together.
The story begins with an unusual school shooting. Initially it appears that one girl, unhappy with not being awarded the lead in the school's musical and in addition having her choice of play thrown out has shot the girl everyone loved, all girls aspired to be, who had been given the lead and perhaps who's parents influenced the choice of play. Then turned the gun on the third girl then herself. But, there is some curious evidence that doesn't fit the scenario. Little by little who the players in this story are, and there are many, come to light. Rumors flying around the school and community keep the investigators searching for the truth or as close to it as they can. Is the right girl being blamed for the shooting? Perhaps the girl who had only received a minor wound to her foot was responsible, her shoes had not been recovered at the crime scene. Would the presumed shooter come out of a coma and be able to tell investigators her story? Just as we slowly learn about the many characters we slowly get as close to the truth of the shooting as possible. We are left to make our own decisions regarding a number of things including pretty rich girl Kat's father's influence on the events and just how a pig slaughter went down and who the unknown witness to events really was. We also find out that however perfect this powerful trio seemed, they were far from perfect and quite good at subtle bullying and promoting cliques.