How large does a run have to be to comfortably accommodate 3 laying hens?
Minimum would be 30 sq ft. but more is always better for a run. Even for just three I'd do at least 10x10. Depending on what materials you're using you might want to size it by the material size, i.e. wood is usually in 8' lengths so 8x8 for a frame is more efficient than trying to cut it down to something like 5x6, plus the birds will appreciate the extra room.