This pattern is very similar to the standard Factory except that the dropped ball is Caught under the arm.
Hold one hand straight out in front of you as it would be if you were juggling a cascade. In the other hand take one ball & hold it at head height & slightly outside of your opposite elbow. Drop the ball & it should fall down past the outside of your elbow. As soon as you have released the ball, swing your hand down & underneath the other arm to make an Under the arm catch. Practise this dropping with both hands.
Now take all three balls & juggle two in one hand columns style, while holding the third ball. As you juggle think of the two balls, the one closest to the centre of your body is in the inside column & the other ball is in the outside column.
Watch the outside ball, when it comes to making a throw in the outside column lift the ball that you are holding in your other arm straight up so that it mirrors the throw. Then as the ball in the outside column peaks & begins to fall, carry the held ball straight across over the top of the pattern to a point just past the elbow of the juggling hand. At the same time as you make the next throw up through the centre of the pattern, drop the held ball straight down. Swing the hand that dropped it round & down underneath the juggling hand & catch the dropped ball. Also catch the inside ball of the two hand juggle, if you freeze the pattern at this point you should be holding two balls one in each hand with your arms crossed & the third ball should be peaked at eye level. Now press play again & uncross both arms as if performing a Crossunder. Throw the ball from the inside column up on the outside of the ball in the air, this will be the new outside column. Catch the other ball (which is now in the new inside column) & just hold the dropped ball out of the way & continue juggling two in one hand columns.
Do the same again but after catching the dropped ball uncross both arms & carry the dropped ball straight up to mirror the ball thrown up in the outside column & start the drop move again. Throughout this pattern the dropped ball remains the same all the time. Remember to only drop the ball when you have finished the horizontal carry, if you don't the movement of your hand will throw the ball out sideways, stop then drop. Practise until you can keep the pattern going consistently & on both sides.
You can take what you have just learnt & make it into a symmetrical pattern by switching sides at the point where you unross your hands. Instead of carrying the dropped ball out of the crossunder, carry the ball that was in the inside column & throwing the dropped ball up on the other side of the pattern.
This trick mixes well with Columns, Factory, Crossunders & Reverse Crossunders.