Basically it comes down to writing as many different threesome-centred stories as possible. The catch is that the next writer should take one of the characters from the previous story, have them wake up and realise it was all a dream before writing their own smut scene (or one-shot, if you want actual plot) using that character and two others of choice.
Example: If you’re the sixth poster, you have to use one of the three characters from chapter five.
- crossover (according to the rules of Twisting the Hellmouth)
- threesome (slash or ...