The Extended Box, also known as
the Double Box, is a three ball variation of the Box in which the pattern is
juggled on each side of your body. This is achieved by keeping one of the
vertical throws at the center of your body, and then alternating the
position of the second throw. The Extended Box is probably the easiest Box
variation, so as long as you are already comfortable with the standard
pattern, you should have no problem with this trick.
To learn the Extended Box, there is really only one intermediate step to practice. Start by juggling the regular Box pattern, and then choose a vertical throw to make slightly higher. (This is purely to give you more time when practicing. In the actual pattern all throws should be the same height.) As that ball is traveling through the air, shift the position of your hands such that the hand that didn't make the higher throw is now underneath it. As that ball descends toward your hand, make a vertical throw from your other hand coupled with a horizontal pass from the hand underneath the high throw. This pass will clear space for your hand to catch the high throw ball. You will then catch the other vertical throw with your other hand.
Practice this on both sides. To run the full Extended Box, simply make another vertical throw from your hand in the center, and then shift your hands back to the other side of your body. To run the pattern at its full potential, try to keep the center throw as vertical as possible, or else the trick may begin to look more like an alternating Shower.