Talking It Over by Julian Barnes

"'Talking It Over' is the story of three Londoners: one woman, two men, all near the age of thirty (give or take a few years), who, despite their basic differences - Gillian is reserved, Stuart is a bit pedantic, Oliver leans toward flamboyance - mark the three corners of an orderly triangle: Gillian and Stuart have just been married, Stuart and Oliver are longtime friends, Oliver and Gillian get along fine. Straightforward and simple. Everyone in their places, getting along, going along..."--Goodreads