Juggler's Tennis is where one ball is thrown back & forth Over the top of the other two, like a tennis ball being played over the net. A different coloured ball will definitely help you keep track of this pattern. If you have one red ball & two blue, just make every throw with the red ball over the top.
When learning this trick it is easy to lose control of the 'tennis' ball. You may find yourself throwing it higher & higher until it goes beyond your reach. The throw does not need to go beyond head height. Just relax & wait for it to come down & always make your catches at waist height. Throwing forwards will probably crop up again & can be corrected using the same tips used for the Cascade.
'Tennising' will come up in more complicated patterns such as Cross Arm Tennis & Burkes Barrage so learn it well. It can also be done with Under the arm throws where the tennis ball goes underneath the net & this is used in Mills Mess & Rubenstein's Revenge. But after learning this trick you might as well proceed straight to the Half Shower.