Plot is defined as all the literary elements that tell the story. These include, an inciting even (incites the problem, or conflict, to be solved); rising action leading to the moment of decision; climax, the moment of decision that determines how the problem (conflict) will be solved; falling action that leads to the final resolution; resolution, in which the problem (conflict) is finally solved. This structure allows for subplots that are complementary or complicating storylines that develop in relationship to the plot. When it comes to the conflict, there can be a series of conflicts that add complication or suspense or intrigue to the main conflict. Since I'm not exactly sure what you mean by "main plot" in reference to the straightforward comedic novel Three Men in a Boat. I'll describe the one and only plot.
In Three Men in a Boat, three friends are so bored that one even visits a doctor to give him a barrage of symptoms that could indicate a diagnosis of many things including typhoid and cholera. The doctor wisely prescribes him a pound of beefsteak and a pint of beer to be taken every six hours. The three friends then decide on a vacation in a boat. Each friend has an area of expertise. J. has expertise in choosing food provisions because he had once chased all passengers from a crowded train compartment on account of the cheese he was transporting. Harris has expertise in getting around places because he once bought a map to Hampton Court Maze and proceeded to study it with care and get himself and a group of blindly trusting strangers lost and trapped in the Maze. Which, by the way, is the major symbol of the story: Humans all too often become trapped in inertia by being stuck helplessly in the befuddling maze of life.
On their journey they fail at everything from finding the right train to take to their departure point for their boating trip--but then again, no railroad official knew the right train either (must have been stuck in that maze, too...)--to bungling the jobs of setting the night shelter and boiling water for making tea to making muddied messes of their laundry by washing it in the river silt. They wisely decide to call their trip short, take the train back to London (they found this train without having to bribe an engineer), have supper, order dinner, entertain themselves, eat again and then they become content. Finally, they have gotten out of their maze in which they were standing wondering and waiting and have found action and decisiveness leading to French cuisine and entertainment. Some of the best things in life.
The book was initially intended to be a serious travel guide, with accounts of local history along the route, but the humorous elements took over to the point where the serious and somewhat sentimental passages seem a distraction. Much of the praise for Three Men in a Boat centres on how undated it appears to modern readers, the humous is still fresh and witty.
The three men meet one evening and discuss their various illnesses. They conclude they are all in need of a holiday and decide on a river trip on the Thames, during which they will camp, notwithstanding J.'s anecdotes regarding previous mishaps with tents and camping stoves.
They embark the following Saturday. They are unable to find the correct train so they bribe a train driver to take his train to Kingston, where they collect their hired boat and start their journey.
The remainder of the story relates their river journey and the incidents that occur. The book's original purpose as a guidebook is apparent as the narrator describes the passing landmarks and villages and reflects upon historical associations of these places. He frequently digresses into anecdotes. The most frequent themes are river pastimes such as fishing and boating and the difficulties they present to the inexperienced and unwary.
The book includes classic comedy set-pieces, such as the story of two drunken men who slide into the same bed in the dark, other amusing incidents are the plaster trout and the making of Irish Stew from leftovers in Chapters 14 and Chapter 17. My own favourite “funny” part is the scenewhere Black’s Medical Dictionary is consulted and by the end J decides he has every single illness in it with the exception of tennis elbow and “housemaid’s knee”