Step 1. There are also outside throws when doing two balls in one hand.
Step 2. By making every second throw an outside throw, you get columns. You can also switch between clockwise and counter-clockwise rotations.
Step 3. Take a third ball in to your empty hand. You can move it around as you wish. Here are some variations, but you can also invent your own pattern.
Fake columns,
Yo-yo,
Around,
Side to side.