Based on the provided summary, the theme of the story could be centered around:
Facing One's Fears: Claire has always been someone who double-checks everything and is constantly on edge due to her anxiety and worst-case scenario mindset. Now, she's in a situation where her worst fears have become a reality. The story could explore how Claire confronts these fears and whether she overcomes them or succumbs to them.
Trust and Suspicion: The situation in which the three girls find themselves could lead to alliances, betrayals, or mistrust among them, highlighting the fragility and complexity of human relationships under extreme circumstances.
Reality vs. Paranoia: Claire's predisposition to worst-case scenarios might blur the line between reality and her anxieties. This could delve into the theme of how our perceptions and fears can sometimes distort our understanding of actual events.
Without reading the full story, it's difficult to pinpoint an exact theme, but these are potential themes that could be explored based on the given summary.