Although most commonly called the box, this trick is also known as the See-Saw, because two balls alternately rise up & down on either side of the pattern while the third ball is thrown horizontally back & forth underneath them. It is closely related to The Shower & provides an easy way to change between clockwise & anticlockwise showers.
Start with two balls, one in each hand, held at waist height & shoulder width apart. Throw one ball straight up to eye level. As it peaks throw the other ball in the opposite hand straight up to the same height. As that peaks throw from the first hand again & keep going. Work on getting nice even throws, straight up & straight down, you shouldn't have to move your hand at all to make a catch. Listen to the rhythm too, you should be able to hear the balls landing in your hands, catch, catch, catch, catch... the length of time between each & every catch should be the same.
When doing this exercise try not to switch your head from side to side to spot each ball as it peaks. Focus on a spot in the middle of the two columns & see the balls in your Peripheral Vision. Stare straight ahead at all times.
Next do exactly the same but click your fingers of the free hand underneath each throw. The click should be at exactly the same time as the ball in the air peaks & the opposite hand makes an upward throw. Keep the throws to equal heights & keep the even rhythm.
Now with three balls you are going to do the above but instead of clicking your fingers make a horizontal throw with the third ball.
Start with two in one hand & one in the other, make the first vertical throw from the hand holding two balls to just above your head to give you a little bit of time to shift the second ball in that hand. Roll it forward & hold it with the fingertips. As the thrown ball peaks & starts to come down throw from the other hand up to eye level & make the horizontal throw simultaneously. Don't pass the horizontal ball back immediately or you will probably end up with two balls in one hand, wait for the ball above it to peak & for the first ball to be caught before throwing the next pair of throws.
Build up in stages, start with the single throw, throw a pair & stop. Then throw two pairs, then three & so on. It is very easy to fall out of the rhythm in this pattern & start making the pair of throws off time, that is not simultaneously. As you release a ball in a vertical throw, keep your fingers spread out & just turn your hand in a little to catch the horizontal throw. Make each horizontal throw from the fingertips, pull your wrist back & snap the ball into the opposite hand without moving your arm.
To get into a Box from a cascade simply throw a single ball straight up as in Two in one hand instead of throwing a normal cascade throw & go from there. Try stopping for a beat by catching & holding the horizontal ball, then catching the vertical ball in a Fork/Palm Stack before continuing or going into some Fork tricks. Also try catching the vertical balls Penguin style or making the vertical throws Over the Shoulder.