My Polish pen is 12 x 16. And a big tree, and netting over it. But I plan on adding more Polish. I don't think Polish need as much room as standard chickens because they really don't free range like the others.
12 x 4 sounds okay for 3 Polish.
My Polish had always been in with my standard flock of 20, for 4 years. But I got a Buff Orpington roo a year ago and they would stay in hiding all day. So I made them their own pen. There was also one hen who would actually growl and run after them! The others were fine.
My Polish do much better with themselves. They are more relaxed and happy, and have been laying alot of eggs. I have 3 in that pen. I have 2 Polish still with the flock, and they do well there, no problems.
If you want to try them with the others, make sure they have their bangs cut so they can see, put out some obstacles. And introduce them by having the OTHER chickens come to the Polish pen. Seems that helps.
I love my Polish. I also hatched some mottled houdans, which are like Polish but bigger. Look at feathersite and see if you like them as well.
Oh yea, all my Polish at some point had jumped into the silkie pen and make themselves at home like they belonged there! When there were 5 in there, I had to move them out, LOL