If your site fits all three niches then I'd submit it to the category that's closest to the sponsor's site you're promoting, unless the sponsor site you're promoting is "TeenThreesomeCumshots-dotcom", in which case.....I got nothing.
Maybe if your sponsor is more of a general porn site then submit to what you think is the least submitted-to category for a higher listing? In this case it's probably threesomes.
