My favourite variant of this genre is when the third person in question is also a JAV idol—and even better if she participates, to some extent. Exemplars would be:
HTMS-116 (Miki Sunohara, final scene)
HTMS-096 (Natsuki Yokoyama, 50min)
STARS-104 (Yuna Ogura by Rena Aoi)
STARS-217  (Suzu Honjo...