A shower is a trick where all objects follow each other round in a circle. You've probably already tried the Half Shower. The full shower is exactly the same but instead of throwing the lower ball up, it is thrown horizontally directly into the other hand. Be sure to learn this trick in both directions.
There are two ways to perform the shower. One is to throw alternately, the other is to throw simultaneously.
Start with holding two balls in one hand, this will be the 'throwing' hand & the other hand empty, this will be the 'passing' hand. Throw both balls, one after the other with High Throws to the other hand & click the fingers of your passing hand after each throw, say to yourself, "Throw, click, throw, click, catch, catch." the first throw needs to be high enough so that it lands in your passing hand after the second click, the second ball also needs to be thrown to the same height. Fix your gaze to where the balls peak above your head, practise until you can comfortably make both throws & catches with the clicks in between without having to reach out your arms. The third ball will replace the clicking.
Next, still with two balls, do exactly the same but as each ball is caught, pass it back into the throwing hand. This time say out loud, "Throw, click, throw, click, catch, pass, catch, pass." Don't look down to make the passes, in the full pattern the passes are made BLIND.
To do the three ball shower fit the third ball in place of the clicks. Start with two in your throwing hand & one in the passing hand. Start as you have done with two throws one after the other from the throwing hand then pass the third ball underneath into the throwing hand. From then on just throw & pass alternately. From the beginning you need to say, "Throw, wait, throw, pass, catch, pass, catch, pass..."
Always look straight up at the peaks of the high throws, keep your two hands close together, the less distance the pass has to go the less likely you are to fumble.
This way is a lot faster but looks much neater. The throws are made to chest height & the passes become actual throws rather than just physically placing a ball in the other hand.
The easiest way to learn the simultaneous shower is to juggle an alternate shower & gradually reduce the height of the high throws until they are being thrown to chest height, you will find that both hands are naturally throwing at the same time.
If this doesn't work for you, build up to it in the same way explained above for the alternate shower.
Hold both hands out at waist level, shoulder width apart, begin with two balls in the throwing hand & throw them both one after the other to about chest height so that the first ball lands in the passing hand after the second ball has been thrown. Click the fingers of the passing hand as you throw the second ball.
Before going on to three balls take a little time to practise the horizontal throw. Keep your hands at waist level & shoulder width apart & pass one ball back & forth without moving your hands closer together. The pass needs to be thrown quite hard to make sure it actually gets to the other hand without falling. All throws in a simultaneous shower need to be made with the wrist only.
Now take three balls, two in your throwing hand & one in the passing hand. Throw the first ball then throw & pass at the same time. Concentrate on the peaks of the throws at all times, the passes will take care of themselves. Keep both your hands at the same height. Make sure each throw goes to the same height to keep a nice even rhythm.