I wouldn't say this is a bad idea. But cross training so early, and at a young age can be confusing. Imagine going from one class, and having your teacher tell you to do things one way, and then going to another class and your teacher saying, that no that's wrong and your first coach is an idiot. That would be confusing and frustrating.
The fact is wrestlers and Jiu Jitsu fighters both fight differently. They have different goals in mind. Whereas the wrestler will never fight off of his back out of fear of getting pinned, but the BJJ fighter will fight all day off of his back. Or where the wrestler will happily give his back to his opponent because there are no chokes in wrestling, the BJJ fighter will never give you his back out of fear of being choked. It's stuff like that. If you can learn to combine the best of both worlds, then you'll be a great grappler.
You don't need to worry about incorporating boxing. Boxing revolves around stand up, and wrestling and BJJ won't conflict with that.
Good luck
I think you should start with the wrestling with the goal to go as far as it will take you and to enjoy the journey and the learning. To go into training with the intent to quit after a certain amount of time to start all over in something else regardless of where you may be in your training kind of puts a limit on what you could learn, especially towards the end of your self imposed time limit, it will be a countdown and distract from your focus on training and making the best of what you are learning. Cross training should not be planned in the manner you do it but should come when the student is ready for cross training and when he has the opportunity to do so, not after a certain time limit. Who knows if there will be a good teacher in boxing or Jui Jitsu in three years while you may have an excellent coach in wrestling? Are you going to give up the wrestling then anyways? Putting a time limit on your training is still putting a limit on your training. Not even you will be able to say where you will be in three years with your training, so how can you make a plan like that already?
Why should any school accept you as a student when you are going to leave so soon and not have gotten that far in your training? You will not have even scratched the surface in a good school.
I'd would tell you t find another school. This is not the place for you. Someone else will take your money. You won't know much when you leave but they will have some of your money. It will help to pay some bills.
Stay with wrestling for school. Take boxing lessons as a after school hobby. School comes first. Ufc should be very low priority at your age
Its good.
Lets say if I want to study Wrestiling ( Freestyle ) for like 3 years and then move on to boxing and Jiu Jitsu is that a good idea? I am 13 by the way.
