Throw ball number two away when ball number one is at the highest position. Not sooner and not later. Try this the other way around too. First throw the ball with the other hand and when that ball is at it's highest point then throw the ball you have in your dominating hand. Then try to catch both balls.
Most people succeed pretty easy until here.
Start with two balls in you dominating hand and one in your other hand. Do the following withouth cathing the balls first. Timing is very important at this moment and catching the balls is not important at this point. When ball number two is at the highest point you throw the last ball in the air. That is all there is too it.