The thrid person should be in the relationship, but the story can be set in any time of emotional development: the third is hagging out with the couple, or is a poly or a extra fuck buddie or any other type of relationship.
I would ask no Rachel(in any part), just because I want to read different things.
It can be het, slash or femmeslash.
Oh, I would like if het, both the guy and the girl like/loved the third that can be girl or boy.
Thank you ^^