In other words, does it work like this: I choose what all requests I want to write and send a message to the admin and then the admin will decide (out of those requests) which one is booked or not? =/
Whoever is the admin for the challenge (in this case, it's Phyrebrat) will randomly allocate the stories to the people who've volunteered to take part.
In order to volunteer, you need to make a request of your own. For example, you might want a tragicomic fantasy set in Uganda featuring clowns. You put your clown request up on this thread publicly for all to see, and it will get given to someone in secret, by Phyre. There's no choosing involved (hence it's a secret!). You won't know who gets your request.
So three people can't "choose" the same request. Everybody who puts in a request, gets a request.
However.
Some people (naming no names) like to go above and beyond the call of duty and write more than one story anyway. Somebody last time wrote four of the gits. So write what you want, so long as you write the story you're given
Read post 1 of this thread for the full lowdown, or just ping more questions up here. Good luck with it, and thanks for playing
Edit: you need to get your request in sharpish, BTW - I think Phyre is sending out the briefs this weekend.