Welcome to BYC.
I am sorry your bird is doing poorly at the moment - any significant change in behavior is definitely cause for concern and further investigation. How long have these birds all been together - meaning are the two other hens recent additions to the flock or have they all been together for 6+ months? When you say the two other hens are a "bit scary", what, specifically, do they do? How long has the silkie been refusing to come out of the coop? Is she moving about normally inside the coop - eating, drinking and producing waste normally - or is she sitting in one place all the time? If it is the latter, how is her demeanor? Have you picked her up to assess her body condition?