I would say your third hen isn't broody. Nothing you can do to make her sit.
I would say she's not actually broody. What has her behavior been like for that week? On the nest night and day for a week? Rarely getting off the nest? Protective of her nest? Could there be another reason she's sitting in the nest boxes? Could she be hiding from an aggressive chicken? I'd just let her be and do her own thing, and not try to give her any more eggs.
