I have 2 chooks which are ganging up on the younger chicken,sad to watch. Please help! i need an explanation!?

we had 3 chickens, lovely little ladies who always layed eggs for us. Then on one fateful day, a dog came and nocked one off the block :'( The owner bought another chook to pay for the one which was missing and this one is much younger than the other two. The other two are always pecking at it and chasing it around. Its very sad to watch and i was wondering if this is normal? Will they stop attacking the poor little thing? Are they just establishing a pecking order in the coop? Please answer soon! :D