This is not a very impressive trick on its own, but it is a very good one to learn as it will be needed to learn more advanced tricks like pistons, four ball juggling, half orbit.
There are many variations on this trick, most just slightly different.
STEP 1 : Throw one ball straight up in the air
STEP 2 : When it reaches the top of your box throw the other 2 balls straight up, but slightly to the side so they don't collide.
Variations
Circular
Practice with 2 balls in 1 hand, throw the balls round in a elliptical motion. This is the first step to doing four balls.
Reverse 2 Ball
Use 2 balls in 1 hand. Move right hand over to the left and throw an arc back to your right hand would normally be. Then move your hand further right and throw an arc to where your right hand would normally be.