[Blackmane]

To support what mariolover is saying:
Of course throwing 100 marbles would be different than 1, but I bet you could throw 3 marbles the same distance because you have a lot of energy and momentum in your throw, but not all of it can be taken by the single marble. A bow is similiar: Of course trying to fire 100 arrows wouldn't work, but 3 arrows get almost the same force as 1 because it builds up an enormous amount of potential energy, but in the release, it releases a lot of that energy away because it can only give so much to one arrow. The string will only move an arrow so far, but it has enough energy leftover to fire off several more arrows and releases that through vibrations, heat, etc.

Therefore, add two more arrows to the system and you have the same force in each arrow, and less energy released in different ways.