Three is a good start. Even after many years of butchering I still don't like the process. It's never easy. Just something you have to do. My number one suggestion is to find a method that works for you and don't hesitate. I personally use tree loopers because there's little chance of error and...
I'm guessing there are at least a few that you know you want to cull. Cull them now.
You have 7 cockerels and 25 pullets. Roosters and hens are over a year old. These distinctions are important in your case. Your girls are young and are likely not ready for breeding. They are probably fighting...