Half of Mills Mess, sort of. If you can already do the full pattern you may have already stumbled on this trick by accident. If you can't already juggle Mills Mess then learning this on both sides will hold you in good stead.
Take all three balls & juggle a Reverse Cascade, now throw one ball Under the Arm. Catch the next ball before uncrossing your arms as an Under the arm catch then uncross your arms, make sure the next throw is thrown Over the Top & carry on with the reverse cascade. That is one repetition of Half Mess & pretty much all there is to it.
For the full pattern just concentrate on the hand with which you make the under arm throw & catch with. It should throw one under the arm then one over the top alternately, just say to your self, "under, over, under, over..."
Be sure to learn this on both sides, it will help when learning Mills Mess with reverse cascade insert & the Three Reverses Variation.