Till the Heavens Start the Rain by
rufflefeather [Merlin/Gwaine/Arthur - NC17 - ~4k]
author's summery: On a hot summer's night, Gwaine finds Merlin trying to escape from the heat behind the stables. When Gwaine asks Merlin to go skinny dipping, he doesn't say no, because he has secretly been fantasizing about his Summer Camp horse riding instructor since he was 14. And then Arthur walks into the scene.
why I rec this: Do you really need to ask? *points at summery* It doesn't promise too much, you know! It's just as delicious and hot as you'd expect it to be. The scenery, the tension, the desire,... Actually, I think I'm going to re-read it now... Don't wait up for me.