To inquire about one's preference for anal sex in the third encounter(conversation) with said person. Then, to have anal sex shortly, thereafter. Immediately would be preferable, without any intermediate conversations. Male to male use of this is not condonned, but words are words, so if they adopt it, so be it, but this case is refering to guy to girl; girl to guy; or girl to girl. Unless, the male to male conversation is asking about the said male's preference for anal sex with other women, and no anal sex between the males must follow.
Todd: So are you gonna try to go through the backdoor with Jackie.
Derek: I've only spoken to her twice!
Todd: Bro, just pull a "Back in Three"