I am not fully certain but juggling is moving objects in a predictable path so as to repeat without dropping.

In my understanding of the OP, juggling 'one' is being able to throw an object consistently to another hand without handing it. This is an intentional throw of the ball from one hand to another without "moving" the other hand to compensate.

Throwing from one hand to another, either directly or in an arc, requires the motorskills to move an object consistently while understanding the speed, trajectory, and then moving the other hand to receive (not catch) it as expected.

There are multiple elements at play with 3 object juggling. One must throw an arc toss to the other hand, while holding an object, then throwing the object in said hand to free the hand to catch. In reality you are holding two objects with one in motion - until you get the double arc which is now technically juggling.

Three bodies in motion, two hands that are each making circular or figure eight motions, while maintaining a consistent arc and speed (XY (no Z) + T = arc) where the mind either tracks or forgets allowing the predictable movements work themselves into only tracking one object at a time - by setting it's path and then shifting focus or attention to the next.