I don't think it'd work for the first number of posts, until the story really kicked off, but just for the kick of things, why not have the people who are posting try to actually incorporate themselves into the story. As opposed to random three words for shits and giggles, try adding something sensible as if the story were their own story. With everyone emptying out their creative ideas to make this their own story, that in itself with bring the desired variation and humor for it.
Probably sounds weird and difficult, but I think it'd prove to be entertaining. My two cents. I'm sure I'll donate a few more if something hasn't kicked off already ..