It is not exactly mind, spirit, and body in the form of most trinities discussed in this blog, but remember that this is not only fiction, it is a cartoon. Let's be thankful for what we have.
The three are asleep, but their advertisements, the signs on the wall, tell us that they are ghostbusters, with hourly, weekly, or monthly rates. Business is not just slow, it is non-existent. I should point out that I like my non-existence with a hyphen to distinguish it from nonexistence, implying nothing with the zero removed. We are only dealing with a hyphenated subjective void at the moment.
Anyway, the three dream while the lonesome ghosts seek them out--just because they are lonesome, naturally. Lonesome nothings which don't really exist. The ghosts call them complaining of ghosts in their house, which of course is logical. All three ghostbusters are astonished by the sudden perception of real customers, though really the only customers are ghosts and hence unreal.
They arrive at the haunted house with an assortment of material means to battle spiritual entities: A shotgun, an axe, a club, and a butterfly net. All of which humorously backfire in due course and are used on themselves in several delightful we-have-met-the-enemy-and-he-is-us scenes.
The ghosts constantly mimic the ghostbusters. In this beautiful scene with Goofy and the dresser mirror, Goofy tries to outsmart himself, his mirror image. This is something like the pantomime Groucho and Harpo Marx had done four years earlier in the movie, Duck Soup.
The harder the ghostbusters try to hurt the ghosts, the more they hurt themselves. This reverberates in the sudden climax of the film, as the ghosts laugh at the foibles of the ghostbusters until they begin to take themselves seriously, causing them to fear ghosts too. They then run away (from themselves).
The lesson here is the same as in Peter Straub's classic Ghost Story, which I reviewed earlier this month. The ghosts are us, that part of us we sublimate and deny. The truth is, WE ARE THE LONESOME GHOSTS!