This is the most basic juggling trick for three balls. Perform it in rooms with high ceilings or outdoors. Perform a basic three-ball cascade pattern, which means the balls rise and fall inside of each other's paths, rather than moving in a circle. Then, throw a ball in your right hand higher than the normal ball path, as high as 6 to 10 feet. The other two balls should come to rest in your hands. When the high ball returns to the normal apex of a throw, throw the other two balls back into the mix and continue.
Either leg works for under-the-leg, but the trick is described here for the right. Initiate when a ball hits your right hand. Simultaneously lift your right leg and drop your right hand outside of the leg. Duck your hand under the leg and throw the ball back into the normal juggling pattern. Make the next throw with your left hand higher than normal. This gives you time to get your right hand back up to make the next catch and continue the pattern.
In a normal three-ball juggling pattern, the hand throws the ball inside the path of the other ball in flight. For an inside-out effect, throw the balls in an outside loop into the middle. When the ball hits your palm, dart your hand to the outside of your body and throw the ball up and in towards a point nearer to the middle of your body than your shoulder. It will fall in the middle, where your other hand darts in and catches it. Throw it again around the outside of the other balls. The result is an effect that looks backwards to regular juggling.
You need a hard floor to do this trick; it also requires hard juggling balls, not soft bean-bag style balls. Instead of juggling upwards, bounce each ball off the floor and catch them with your palms towards the floor. Use a normal juggling pattern, with each ball travelling inside, but simply bounce the balls off the floor instead of throwing them in the air. Practice switching quickly between bounce-juggling and regular juggling for a cool effect.