A bit Box-like, because you have one ball being thrown horizontally back & forth & a bit Shower-like because you have balls being thrown one after the other in the same direction & also a bit Tennis-like because the two balls switch directions to go back & forth over the pattern. What a dull name though.
Start with two balls in one hand. Throw both balls one after the other as cascade throws to eye-level, over to the opposite hand. At the same time as you throw the second ball, click the fingers of your other hand. Practise both ways.
Now with all three balls. Two in one hand & one in the other. Begin with two cascade throws one after the other from the hand holding two. At the same time as you throw the second ball, instead of clicking your fingers, throw the single ball horizontally into the leading hand. Stop by catching the first two balls in the non-starting hand. Then do the same back again. Each time say to yourself, "Throw, throw/pass, catch, catch."
When you can do that the next step is to do the same again but this time concentrate entirely on the two cascade balls that are thrown one after the other. Instead of doing, "Throw, throw, catch, catch." do "Throw, throw, catch, throw, catch, throw, catch, throw..." That is, start with two throws, then as you catch the first ball instead of holding on to it throw it straight back as a cascade throw to the other side. Then catch the second ball & as you throw that ball make the horizontal throw. The effect is two balls following each other from side to side, 'tennising' over a single ball that passes back & forth underneath.
When doing this trick make sure that the horizontal throw is a genuine throw, it must actually travel through the air before being caught. A throw looks much better than clapping your hands together. Also try to get the two tennising balls to peak at the same point to keep the rhythm even.