Sudden aggression between 2 chicks?

[Kyochan]

Hello. I have 3 babies in the brooders. They are 5 weeks old and almost to the age of going outside into their grow out coop. However, last night two of them started fighting so bad I had to seperate them. I thought it was one being aggressive but when I put them each with the third, docile chick they are fine. They only fight each other. Now this started very suddenly with no warning (unless I've missed 'signs' in their behavior) I'm unsure what caused this or what to do. I only have the three so if I have to keep them seperated I have no one to put with the lone chick. Any ideas on what started this or how to fix it? Do you think it will calm down when they go in the bigger coop outside this weekend or is it to risky to put them back together?

 

[ChickenCanoe]

Things always go better with more space. They're getting to the pecking order stage and it's compounded by lack of space.