Credit for this trick goes to Michael Karas. This trick is so simple that it will probably take you longer to read this page than it will to learn it.
Take all three balls & juggle Two in one hand in a columns pattern while just holding the third ball in your other hand at waist height. Grip the held ball using only the butt of the thumb & your ring & little fingers so that your thumb, middle & index fingers are free to make a catch. As you juggle notice that you have two columns, one on the inside & one on the outside.
When it comes to making a throw with the outside ball, instead of throwing it straight up throw it Over the top of the inside ball. At the same time as you make this throw, carry the 'spare' ball held in your other hand up in an arc that mirrors the path of the ball thrown over the top. Your hand should meet the thrown ball at its peak, catch it using the thumb, fore & middle fingers so that you are holding the carried & thrown balls side by side in one hand. At around the same time catch the inside ball with the hand you were juggling two with. Now let go of the ball that was thrown over the top (tip - just lift your fore & middle fingers off the top & let it roll down your thumb, this gives the ball a little sideways motion to carry it past the inside column without spoiling the effect of the trick) & carry the held ball back down to waist height. As you drop the over the top ball throw the inside ball up through the centre & resume juggling two in one hand.
That is pretty much all there is to it. Practise the trick so that you pause just long enough to give the illusion that the balls are held together by magnets as opposed to being held by your hand. Practise the move on both sides & alternate between juggling two in the left & right hands. Work up to switching sides on every second throw so that you are effectively juggling Columns & the two outside balls alternate between being thrown & carried.
This trick is also useful for one off stopping points in flowy patterns. For example juggle Mills Mess, throw the Under the arm ball higher than normal & make it curve in towards the centre of the pattern more. Make the under arm catch as normal then uncross your arms & carry that ball up to meet the ball thrown under the arm. Catch the ball so that you are holding two in one hand, pause, then drop it & carry the held ball back down & throw it up under the arm, catch the dropped ball under the arm & continue mills mess.
It also fits in very neatly with the Singapore Shuffle.