Note that the above is just a demonstration and there are many other ways to get it. Both girls of the 3P do not have to be in a relationship, be bisexual (though they still can't be homosexual), or even have low virtue. It has been known to occur with one girl being highest virtue, hetero and teacher, while the other was lowest virtue, bisexual and a student, the male PC had the Class Prez trait. However, all three participants need to have very high love for the other participants for 3P to trigger.