Competition.

It was a smart idea, but the two smaller ones didn't like it that they had to cooperate. It hurt their pride to have to rely on another for victory. But they had no choice. They had to cooperate or suffer a loss. A loss will hurt their pride more than corporating. Besides, there was a lot at stake here. 

Their fight was mostly for fun, but it had serious repercussions for the hierarchy. As proud creatures, they do not like losing, even if it is to their siblings. They may be weak and have only been born for a little over a year, but their need to dominate is ingrained in them. It is the dragon way. 

They stopped fighting when they heard the howling of the wind. All three of them turned towards the only opening in the cave in expectation. Their expectations were not subverted. A massive-scaled creature blocked out the light of the sun and flew into the cave. The three of them sensed the familiar aura of their mother in the creature. So they ran to her excitedly.