Fig, fun, fog are commonly known, but there are many interesting three letter words that are not often used, like fib, fob, fub, fab, fad, fop,
fab: fabulous; wonderful.
fad: an intense and widely shared enthusiasm for something
fid: a square wooden or iron bar
fib: a lie, typically an unimportant one.
fob: give (someone) something inferior to or different from what they want.
fop: a man who is concerned with his clothes and appearance in an affected and excessive way; a dandy.
fub: verb. an archaic word for cheat.