If they're all housed together, it might be beneficial to separate them, or at least move the thin one to a separate enclosure. They do not need company to be happy, and in fact can sometimes be stressed by it. It may be that the thin one is being dominated by the other two, and if so, that could be why she's not eating.
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0.1 Albino Leopard Gecko - Tigger
0.1 Crested Gecko - Pooh-Bear