Based on this question being in the Hamlet category, I believe the question is asking, "What three Shakespeare plays might be more relevant than Hamlet is to ninth graders?"
That's a good question. It's one that curriculum review committees wrestle with too.
I believe that Romeo and Juliet is more accessible and relevant to ninth graders. One reason is that the story is already known by most people. A general understanding of the events of the play is likely common knowledge to most high school students. Another reason for its greater accessibility to young people is the fact that Romeo and Juliet are teenagers themselves. Young readers like to read about characters that are the same age as the readers. Just look at most young adult literature. The characters are almost always under the age of eighteen. The Hunger Games is about teens killing teens. Unwind is about teens trying not to be killed for body parts. Twilight is about immortal teens. Divergent is about teens. The Maze Runner is about teens. Romeo and Juliet is about teens which makes it more accessible and relatable to teen readers. On the whole, Romeo and Juliet is relevant to high school students because they are learning about love and heartache for the first time, and Romeo and Juliet is a play about those very same things.
I think Much Ado About Nothing is a good choice too. My reasons on this one are not complex. First, the play is a comedy. My problem with Romeo and Juliet has always been the fact that it is a tragedy. I don't like tragedies, and my students don't tend to like them either. Why? Because they are sad. Much Ado About Nothing reads like a romantic comedy. It's happy and upbeat through most of the play and loaded with dirty jokes, insults, and bickering. It's relevant to ninth graders for exactly those reasons. That's how they act a lot of the time. It's a fun, and funny play to read; therefore, students are more likely to want to read it because they are having fun with it. Thematically the play shows what can happen to a relationship when there is communication breakdowns. That fits with high school students too that are learning to communicate with their teachers, their friends, and their significant others.
For my third suggestion, I recommend The Tempest. It's a nice mixture of comedy and tragedy, but it most definitely isn't as dark as Hamlet. The Tempest is definitely on the more exciting side of the spectrum of Shakespeare plays, which ninth graders should appreciate. It's loaded with action, fighting, and magic for the boys, and the play contains a love plot as well for the girls. There's tons of Shakespeare style swearing in it too. Obviously high school students are not dealing with shipwrecks and mystical creatures, so the play is not relevant in that regard. The play is relevant because its overall plot mirrors the same plot structure and character type that students are already familiar with. Additionally, it's loaded with love, loss, betrayal, violence, and humor. These are all things that modern audiences can relate to.
If I was really forced to do it, I could probably say that any of Shakespeare's plays are still relevant to modern day audiences. Alan Craven, a modern day literary scholar, would also agree. He writes the following about Shakespeare's plays and their continued relevance.
Humans still experience love, loss, betrayal, war, humor and tragedy, which gives Shakespeare a foothold in modern times.