This is a great trick, High throws & a fantastic pose. This is basically a Shower but one hand is held up high above your head just like the Statue of Liberty.
First take a look at the position without any balls. Stretch one arm up, lean your head to the side, lift your shoulder & lock your elbow. Now unlock your elbow, drop your shoulder & straighten your spine. Your hand should drop a couple of inches & feel a lot more comfortable. This is the position you need to put one arm into. Should you throw a little too far or wide & your arm is fully stretched, there is nothing more you can do. But if you relax a little you can stretch that little bit further & make the catch.
Take one ball in one hand & hold the other arm up in the air. Fix your gaze at the empty hand above you & throw the ball up into it, the ball should peak just above your hand & gently fall down into your fingers. Open your fingers to drop the ball. As you release it bend your wrist to give it a little sideways push so that the ball falls down into the lower hand. BUT DON'T LOOK DOWN! Always look at the hand in the air, the dropped ball is caught Blind. You can judge where it will land by seeing it fall out of the hand. Practise those two movements, in the full trick the lower hand always makes long throws & the higher hand just drops. Simple.
Now take all three balls, hold one up in the air & the other two at waist height. Throw both balls in the lower hand one after the other up to the other hand, as the first ball peaks, wait until the last possible moment before dropping the ball in your upheld hand. You should be able to open your fingers then close them around the next ball straight away. Then a moment after that you need to open & close your fingers again for the second ball. Finish by catching both dropped balls in the lower hand. Then do it again. & again & again. Try to add more throws, as you catch a dropped ball wait for a moment then throw it back up again at the same time as you release from the higher hand.
As you build up to longer runs start to think about accuracy. Is your dropping hand bobbing & weaving to make each catch? If so practise making more accurate long throws. Stare at the point in the air where you want the balls to peak & concentrate on throwing to that point, by willing yourself to throw accurately you will. Sort of a Jedi mind trick. As soon as you can make consistent throws & you can keep your upheld arm almost still you can consider the trick learnt. Then...
Try different throws. Instead of dropping the ball from the higher hand underneath the next one, flick your wrist & throw it Over the top. Throw from the lower hand so that the ball flys into the upper hand from the outside rather than the inside. Throw underneath the dropped ball rather than over it, or throw Behind the Back.