Start with one ball in one hand. Practice throwing it to the opposite hand (and catching it) so that it curves at eye level.
Step2
When step one comes with little effort, add another ball. Put one ball in each hand. Throw the ball from the right hand so that it curves just in front of your face.
Step3
Throw the ball from the left hand just before catching the ball that is in the air. Repeat 2 and 3, switching hands.
Step4
When 2 and 3 come with little effort, add a third ball. Hold 2 balls in the right hand and one in the left.
Step5
Toss one of the balls from your right hand. Toss the ball from your left hand just before catching the ball that is in the air.
Step6
Toss the other ball from your right hand just before catching the ball that is in the air.