NumbersJuggling is an obsession. I personally can only do five balls for a brief moment, although I have seen nine rings.

Numbers juggling requires that your hands, in business school terms, have transitioned from a "job shop" to a "continuous operation." 

With three - maybe with five, if you are really fast, but I am not that fast - there is time to move ones hand to catch the incoming ball, even if you are doing something like Mills Mess, or something like making all three balls move up and down in place while shuffling your hands around beneath them. ("Boston Mess", or "Pistons")

With numbers, all the tosses must be uniform. There is no room for error correction on the receiving side. The hand is a just-in-time operation and when it is time to catch there must be a ball in the appointed place.

Have a nice day.