Currently studying Criminology and was skimming through my criminal code...
159. (1) Every person who engages in an act of anal intercourse is guilty of an indictable offence and liable to imprisonment for a term not exceeding ten years or is guilty of an offence punishable on summary conviction.
(2) Subsection (1) does not apply to any act engaged in, in private, between
(a) husband and wife, or
(b) any two persons, each of whom is eighteen years of age or more,
both of whom consent to the act
So if there's an anal threesome it's against the law, or if the people engaging in the anal sex are under 18 it's against the law