At the beginning of the story, we get a picture of Jerome, the narrator, as being rather lazy, generally quite easy-going, although often irritated by his companions, Harris and George, and given to romantic and somewhat sentimental musings from time to time. By the end of the book, he remains much the same. We can see this by looking at the final chapter.
In the last chapter of the book, it begins to rain persistently, spoiling the three men’s trip. Jerome briefly launches into a sentimental aside about the dullness of Nature when divested from her ‘lifeblood’ of ‘sunlight’ (chapter 19) before switching back to a more humorous and everyday mode - something he often does throughout the book. He recounts the dismal evening that they spend in the boat in the rain, taking the opportunity, as so often, to make sly fun of himself and his companions, for example when he details George’s attempt to provide some entertainment on a banjo.
Jerome does not really change or develop in character in the course of the book, and neither does Harris, or George. It is not the kind of book which is concerned with the development of its characters; it remains, in essence, a light-hearted travelogue interspersed with hilarious incidents and anecdotes, as well as some more sentimental reflections by Jerome, as already mentioned.
The only way in which the three men appear to change is in their attitude to life in a boat; they decide finally that they’re better off back on dry land. Harris proposes a closing toast: ‘Here’s to Three Men Well Out of a Boat!’ (chapter 19). This is only change in them worth remarking on, and it is not a deep or serious one; it simply serves the amusing plot.