Rolling a ball along the arm is a new type of trick. Most moves in these tutorials are classed as Toss Juggling, that is balls are actually thrown & caught. Rolling balls around your arms & hands is a branch known as Contact Juggling which is very graceful & beautiful to watch. In contact juggling the balls remain in contact with your body. Contact juggling was made famous by the film Labyrinth where Michael Moschen played the arms of Jareth the Goblin King & performed the hypnotic moves with crystal balls. For more about contact juggling take a look at www.contactjuggling.org & www.contactjuggling.com.
Because the balls have to roll contact juggling needs to be done with solid balls. This trick is easier to learn with bare arms.
Take one ball & hold it in a Fork. Raise your arm to chest height. Keep it parallel to your chest with the ball at the centre line of your body. Tip your wrist back to start the ball rolling towards your elbow, but rather than rolling the ball over your arm, move your arm underneath the ball in the opposite direction to the way the ball is travelling. Guide the ball along your arm with small movements towards or away from your chest, let the ball fall off the end at the elbow, you'll probably need to pull your arm in to make sure it falls off to the outside rather than the inside. Catch it in a fork with the other hand & then repeat on the other arm.
Once you can do that smoothly putting it into a three ball trick is simply a case of juggling, stopping in a Fork/Palm Stack, moving your arm into position & rolling away. Rolling the balls from elbow to hand is also possible as is rolling from hand to elbow & back to hand. There are unlimited possibilities for combining contact juggling with toss juggling.