Thank you for asking. I'll be posting a "how does claiming work post" and will address this. But to answer you now, each prompt can only be claimed once, the second and potentially third (unlikely) rounds will be for artwork not claimed in the first round.

The FAQ says:

What happens if the number of authors doesn’t match up with the number of artists? We will do several rounds of claiming so that EVERY piece of art has a fic in the event that the art pieces outnumber fan fiction writers. If it's the other way, we may open up the pieces of art so that more than one fic writer is assigned to the same piece of art. Either way, everyone who wants to participate will get to participate.

In this instance, the number of art prompts exceed the number of writers, so writers may want to challenge themselves and write stories for two prompts. I've been running Reverse Bangs for several years in other fandoms and I haven't seen a situation where a writer is locked out of all three prompts if they get their request in on the day the prompts open for selection.

But if you are locked out, please keep your eye on the comm. There will be something happening after the challenge closes ;) .