Each arrow is one of you Alone you are weak (arrow break) But if three of you go together then you will....(3 arrow break) ....Still weak !! That is why 200 of you must go together!!
I don't get it for why he had to destroy the arrows.
It's a reference to the 3 Arrows Motonari Mori used to teach his sons the lesson of teamwork and solidarity using the 3 arrows to represent his 3 sons. He basically did what was attempted here saying one arrow will break but 3 together are strong.
But here, a servant's strength is clearly too much for mere arrows so DEmiya can't use the analogy.
