In Three Men in a Boat (to say nothing of the dog), each character stands alone in their absurdity.
J is the narrator of the story. He, like the others, is middle-class, dissatisfied with his life, and living in London. He likes history and literaturd and is given to daydreams of delusion that sometimes get him into trouble if he doesn't pay attention. He is a staunch defender of the right to the public to enjoy the river and especially takes exception to noisy steam boats.
George is a bank clerk who also is living in London and hates his job. Like J he is in mid-life. It is his idea to go up the river in a boat since he already does some boating. He does not have a good sense of time, and is not very practical when it comes to things. He brings along a banjo, thinking he will learn to play it on the trip by reading a book on it. He is the one, at the end, that suggests they just take a train home.
Harris is from the same demographic as the other two, and dissatisfied wtih his life as well. He likes to drink alot, as evidence by the scene where he fancies that he's being attacked by swans. Harris has a temper, which the others are used to at this point. They simply let him rant and rave himself back to reality.
Montmorency is the dog and should be mentioned. He is often a point of stabilization for the men in that Monty knows exactly what he likes, doesn't like, and offers a sensible way to view life.