Thanks for the welcomes. You say you can't see it happen, but I think the situation between the two is very much like it is, for example, for Humphrey Bogart and Katherine Hepburn at the beginniing of "African Queen". I don't think it would be an easy relationship, either as ménage à trois or a lesbian affair, and as a lesbian affair it would always be a side affair for Angua. But how would Carrot react?
And a ménage à trois can happen more or less accidentally. Have you ever read "Castle Gripsholm" by Kurt Tucholsky? In that book a ménage à trois comes to pass because the two women lie in bed solving a crossword puzzle, and the man, who is in the adjoining bedroom, comes over to help when the two women start arguing about solving the puzzle.
"Castle Gripsholm" is a recommended read, by the way; a very funny book (Tucholsky was one of the leading German satirists of his time), but with serious parts too. I read it in the original German, but I know there is an English translation around.