In ordinary use, these three words (especially the first two) are interchangeable in their meaning of "prevailing custom" and "accepted style." Thus one can refer to someone's fashion in dress, manner of dress, and mode in dressing. Slight differences in meaning do exist, however. Fashion may be defined as that which sets apart (distinguishes) the manners, dress, and habits of a group or period of time: "That hat would have been in fashion thirty years ago." "Dipping snuff is not the fashion of today's tobacco users." Manner is more often applied to the actions and behavior of individuals: "This hostess has a gracious manner in welcoming guests." "Ned's table manners could stand improvement." Mode has to do with a method of acting, with form, with a way of doing something: "Solar heat is a new mode of warming buildings." "Jack tries hard to keep up with the latest mode in everything he does." Approximate synonyms for fashion, manner, and mode in the meanings indicated here are style, vogue, and fad.
Dictionary of problem words and expressions. Harry Shaw. 1975.