A pattern where all the balls follow each other in a circle & the arms cross & uncross quickly underneath the throws. This is a good start if you want to learn Mills Mess (& believe me you do). This trick is sometimes called the False Shower & the Windmill.
Go straight into this one with all three balls. Juggle a cascade then throw a ball from one hand Under the Arm. Notice when you throw a ball under the arm that you automatically move that arm inwards & down so that it is out of the way. Continue that dodging movement into a full circle so that your hand ends up on the outside of the ball thrown under the arm.
You might find it helpful to read this description & mime the movements without using any balls. During this move one arm throws under the arm, I call this the THROWING arm & the other does some carrying so I call it the CARRYING arm. As you swing your throwing arm to make an underarm throw, the carrying arm moves in & down to get out of the way. As the throwing arm throws straight up the carrying arm continues the circle underneath the ball thrown under the arm while it travels upwards. When it peaks the carrying arm needs to throw the ball it is holding Over the top of the under arm ball.
Practise this move on its own, juggle a cascade throw one under the arm, carry, throw over the top & resume a cascade.
Now juggle a cascade & throw every ball from one side under the arm. Pick a ball & every time it goes under the arm make the carry & throw over the top of it. Work up to carrying around every ball thrown under the arm.
When you can do that you are 'technically' juggling a spiral pattern, but for a 'true' spiral the balls have to follow each other around in a circle. So instead of throwing the underarm balls straight up you need to reach under to the opposite side & throw them back to the same side. The underarm throws should follow the same path as the over the top throws. That is if you throw with the right hand, reach under the left arm & throw from the left side so that it is caught on the right hand side.
Remember that every ball is thrown from one side & caught on the other. All the time both hands catch on one side, carry & then throw from the other. One hand will always be moving left & the other always moving right. As you carry keep your arm movements nice & circular, make all the under arm throws with lots of scoop.
Be sure to learn this trick in both directions, clockwise & anticlockwise circles.