What can I do about my male rats fighting?

Ive had two boys Max and Hershey. I dont know for sure how old they are but I can guess they are at least 4-6 months old. Im guessing closer to six months. Max and Hershey have always play fought but never anything serious (except for when we first got Hershey. They were bought about a month apart.) Max has always clearly been the dominant of the two. My husband and I recently got another rat Darwin. Hes 3 months old. The first day we got him they weren t very happy about it but they quickly accepted him. A few days before we got Darwin max got sick and had an obvious lack in energy (hes been to the vet since and is now all better. Back to his normal self). We ve had Darwin for about 11 days now. And Hershey has recently started being aggressive toward Darwin and now to Max. Yesterday I had to break up a fight between max and Hershey that ended in me getting bit and bleeding. Hershey sat on my shoulder shivering for a good ten minutes. Im not sure who started the fight. I didn t know if maybe adding a new rat upset something in Hershey or if he took the opportunity when max got sick/weak to fight for dominance. Any suggestions or advice? Update: We let them go on their business as usual. We didn't make them sleep in separate cages last night. I feel like maybe Max was trying to reclaim his place as alpha? Im not sure. Do you think this will happen again? What can I do if it does, i dont want them to have to live in separate cages