Put 2 balls in one hand and 1 in the other. In the hand with 2 balls, toss them up and down in columns as you learned in Step 2. In the other hand, toss the single ball in sync with one of the two balls in the other hand. Now you have 3 parallel columns going. Practice with 2 balls in the right hand then 2 in the left so you become adept at throwing the balls from either hand.
Now practice moving the third ball (the one out of sync) from one hand to the other or from the inside to outside position without stopping. You can do this by throwing the third ball over the others to the new position.