If the birds are being entered in a Production class, they will most likely be judged on how well they will lay/are laying eggs. The judge will check their width between pubic bones, distance between the end of the keel bone and the pubic bones, bleaching of body (how much yellow pigment has been lost and placed in egg yolks), condition of feathers and body, and general health. These are all indicators of laying ability.
It is unclear whether these three pullets are entered separately or if they are a pen of birds. If they are a pen of birds, you want uniformity. Pick the three that are closest in color, body size, and stage of lay. Choose the widest, deepest bodied, best-feathered birds that have lost the most amount of pigment. You don't want an excessively fat bird, either.