I would wait a year until your horse is four to do anything besides light flat work. Your horse is still growing and developing internally. Jumping him now might put his joints at greater risk for injury in the future. After that, I would suggest boarding him somewhere with a trainer and beginning lessons with them then! Hope this helped!
Is it possible to bring your horse to a trainer instead? I personally wouldn't try to teach yourself to jump when on a 3year old because they need a lot of guidance and confidence from the rider when they're young, and a bad experience jumping at this age could be problematic later on!
Watch videos on YouTube? Some are really good and just start with pole work and work your way up to cross rails and then verticles. Also read "Training Showjumpers" by Anthony Paullman? I think? Really good book with great exercises for young horses and riders learning to jump