Make sure they are close to the same size before introducing them. They don't have to be exact but you don't want their lives to be at risk. A lot of people put a wire barrier between them for a couple weeks so they can see each other but not access each other. I just let all three of my flocks go and they worked it out.
Someone gave me the advice to just make sure they have a place to run away if they back down from a challenge. That's important. They got a little pecky with one another but usually when one ran away the other didn't chase it too much.
It's a process. I mixed mine 3 or 4 weeks ago and they are just now starting to mingle and even now it's not uncommon to see them with their individual flocks.