In order to juggle 3 balls, first start with one ball in your right hand throw and the ball straight into the air, like this.
Repeat it several times and try to keep the elbow tucked in parallel to your body. Let the ball come to you.
Break the ball's fall as you catch it.
Then try to do the same thing with your left hand.
Then shoot the ball between the left and right hands.
At first, track the ball with your eyes.
Then, you're going to gently throw the the ball from right-hand to the left hand while making a rotation movement just in front of the belly button.
Slowly but surely, you can speed this up so it becomes a more fluid movement.