Put one arm over the other so that one elbow is resting in the crook of the other. Now do a Reverse Cascade with your arms in this position.
Practice with one ball throwing from the outside to the inside. Because your arms are crossed you don't have much scope for scoop so hold your arms so that the fingers point out to the side, look down & your arms should make the shape of a flat 'X'. Now you can throw by scooping your wrists rather than your arms. Catch at the base of the palm & throw from the fingertips. Throw slightly forward so that to make the catch you have to close up your arms a little as if they were a pair of scissors. Throw with the scissors fully open & catch with your forearms slightly closed.
When juggling crossed armed with all three balls at any one time the arm which is about to catch will be 'closing' (moving away from you), while the arm that is about to throw will be 'opening' (moving in towards you).
Learn this trick both left over right & right over left. Once you can do this trick you might like to learn The Three Reverses.