Matt's Mess, also referred to
as the Simultaneous Mills Mess, is a variation of the Crossunder pattern in
which the center ball is thrown from the outside and travels between the two
vertical balls before being caught in the middle. Essentially it is a fusion
of the Crossunder and Reverse Infinity, so before attempting Matt's Mess it
is important that you already be comfortable with both of these patterns.
To begin learning learning Matt's Mess, start with two balls in your dominant hand and one ball in your non-dominant hand. Cross your arms and make the same pair of throws you would normally perform in the Crossunder and then, as those balls begin to descend and your arms uncross, throw the third ball such that it will travel from the outside to the middle of your body. Catch the two vertical balls and then make another set of crossed-arm throws (in the opposite configuration). The center ball should travel between these balls and then be caught as you uncross your arms by the hand that was crossed under your other hand (just like in the regular Crossunder). You will then catch the two vertical balls.
As shown, practice this on both sides. This is pretty much the only intermediate step you really need to practice. To run the full pattern simply make another outside-to-middle throw with the center ball and follow it up with a set of crossed-arm vertical throws. To get the greatest effect out of Matt's Mess you will want to juggle it low and compact, close to the size of your regular Mills Mess pattern.