Multiplex means that we throw more than one ball from one hand at the same time.
Start with two balls in the right hand and with one in Your left hand.
Toss the ball from Your left hand and when the ball is at the peak of its trajectory throw the other two balls from the right hand above the first one.
Now right (empty) hand catches the ball which we tossed first and immediately throw it back to the left hand.
Then we catch both balls tossed at once (each to other hand) and then we catch the last ball in the left hand.
Now we are in starting position again with changed sides and we can start to toss again.