Hi, welcome to the forum! Glad you joined!
Fourteen months ago, I got three Silkie chicks that were a day old and guaranteed hens. Well surprise! They were all three roosters
Thanks. That age is important information. They should be acting like mature roosters, not immature hormone driven cockerels.
One way many people stop cockerels or roosters form fighting over the girls is to create a bachelor pad. That's where you put nothing but boys in a pen. With no girls to fight over they are often very peaceful with each other. They still have a pecking order but it is often not as violent as if there were any girls around. That's what you accidentally wound up with.
What will happen when you add girls is a huge question mark. There is always a possibility that the boys will continue to get along. It happens, just not a lot. It really helps when you have a lot of room for them. I'm not talking about that 4 square feet in the coop and 10 square feet in the run, I'm talking about real room where they can get out of sight of each other. But even in relatively tight spaces it can possibly work. You do not get guarantees with chickens and their behaviors.
What is more likely to happen is that they fight over the girls. This could be a fight to the death or they may reach an accommodation on how they work together to take care of the flock. If you do bring in some girls I strongly suggest you have a Plan B ready in case they need to be separated quickly.
What hens should I get for them and how should I incorporate them together? Should I get standard more sturdy hens? I'm worried these three bantam boys could tear up a smaller bantam breed hen.
Why do you want the girls? What are your goals with having them? Eggs, hatch chicks, eye candy? Something else? It's hard for me to suggest certain breeds if I don't know why you want them.
You can read a lot on this forum about ratios of boys to girls and all that. I personally don't believe in those ratios relative to over-mating. Those ratios are generally about insuring fertility in a hatchery setting, not how we typically manage them. I've seen some pretty low ratios of girls to boys do fine as far as over-mating. But I've also seen some pretty high rations have issues. To me it has a lot more to do with the personality of the individual chickens than raw numbers. No matter how many girls you have, the boys can select a few favorites and wear them out while the rest go unscathed. Same thing relative to boys fighting over the girls. The ratio has little to do with it. Still, I'd get more than a small handful. Your odds of success are better.
Should the Hens be in a seperate run to start with and they can sleep together at night? Each run has it's own door that leads to the coop in the middle. What quantity should I get? Or should I just let them be by themselves for the rest of their lives?
Again, why do you want the girls? I look at is as "If it ain't broke don't fix it". The odds of things remaining peaceful remain high if you don't changed since they are over a year old. If you bring in some girls the odds of things getting a lot more dramatic go way up.