I'm afraid I can't say if they're like chickens because I could probably write everything I know about chickens on a bar napkin. As far as being accepted back into the flock, I can't guarantee it, but it's very likely she will. When you're going for purebreds, you have to separate your flock during breeding season or you'll end up with who knows what. I imagine that's the same with chickens. It's also fairly common to be forced to do segregation because of aggressive behavior, almost always of a drake or drakes. You just let them mix back together and keep a very close eye in the beginning to make sure she's not being attacked by the others. That warrants a slower reintroduction.
When you say 3 weeks being away, are you talking about until hatching? Mallard eggs take around 4 weeks to hatch. During that time, you want to softfly talk or sing to the eggs when you're in the area. I don't know if people do that with chickens, but with ducklings, it helps them imprint. They learn to recognize your voice while they're still in the eggs. That doesn't mean you have to continually stick your head in the coop, which could be upsetting to the hen. Just do it whenever you're in the vicinity. I often can't think of anything specific to say, so I sing or recite a poem, just anything that allows them to get familiar with your voice.