I have Dorkings myself and they are pretty good flyers. They wouldn't have any trouble with a 3' door. My nest boxes are about 3' off the ground and my Dorking lay in them just fine. My Dorkings also roost almost 6' off the ground.
I have a couple of Wyandotte roosters that free range. They have some size to them now, but they still sleep in the rafters of our barn. So I suppose they don't have issues flying either. LOL. Our rafters are probably 8-10'. They sleep up there with our Black Giant rooster.
The silkies need ramps for just about anything. They just don't get any lift with those feathers of their's.
Oh yeah, my hens make their daily 3' jump/flight with their wings clipped. I thought that would slow them down when I was trying to keep them in an uncovered enclosure.. Boy, was I wrong. LOL
-Kim