adding one is tricky as the other two might gang up on her and be mean to her. You also run a risk of bringing in diseases when you add adult birds to your flock. If you do want to add one, I would recommend you add her to the coop at night
Do u think it best to put them together at night and separate by day for a while? Should they be able to see each other but not reach the others, how long for? I guess really I will just need to keep an eye.
Depends on a few things.. Is your new addition younger or older than your existing hens? How od are they? Are they the same breed? Small flocks can be the most difficult integration. I would try a few introductions through a cage for a few days then put her on the roost at night when you do introduce. I find the easiest hen to introduce are ones that fall average to slightly higher in the pecking order among their peers. I know not always possible to have lots of time to chose but sometimes these smaller flocks need careful selection. Also, selecting a different breed then your current hens can be a issue as I find same breeds tend to stick together.