This is a complex film that explores the effects of Fordist and Taylorist modes of industrial capitalist production on human relations. There are constant references to assembly line production, where workers are treated as cogs in a machine, overseen by managers wielding clipboards, controlling how much hair the workers leave exposed, and firing workers (Stanley) who meet all criteria (as his supervisor says, are always on time, are hard workers, do good work) but who may in some unspecified future make a mistake.
This system destroys families - Stanley has to send his father to a nursing home (where he quickly dies) after Stanley loses his job. Iris' daughter is a single teen mother who drops out of high school to take a job in the plant. References are made to the fact that now, with declining wages, both partners need to work, the implication being that there's nobody left at home to care for the kids. Iris' husband is dead from an illness, and with the multiple references in the film about the costs of medical care, the viewer must wonder if he might have lived with better and more costly care. Iris' brother in law gets abusive after yet another unsuccessful day at the unemployment office when his wife yells at him for buying a beer with her savings instead of leaving it for her face lift and/or teeth job (even the working class with no stake in conventional bourgeois notions of perfection and beauty buy into them). The one reference to race in the film is through a black factory line worker whose husband is in jail (presumably, he's also black, and black men suffer disproportionally high incarceration rates). She remarks that he, like her, "is doing time" - her family is composed of a prisoner and a wage slave.
Stanley, however, still believes in human relations and is therefore for most of the film outside of the system of Fordist capitalism. He cares for his father in spite of the fact that it was his father's traveling salesman job that resulted in his illiteracy - he has not yet reduced human relations to a purely instrumental contract, as Iris' brother in law does (suggesting that he "married the wrong sister"). He does not, as Iris says, conform to the work-eat-sleep routine of everyone else; rather, he uses technology and the techniques of industrial production in an artisanal and creative way, in a sort of Bauhaus ideal. This was the dream of early modernists and 1920's socialists (such as the Bauhaus) - to use technology to provide for all basic needs, allowing for more free time for creative human work and fuller human relations. He is also outside of traditional gender relations. He cooks, he cleans, he cares for his family, and he knows how to iron. Iris, on the other hand, lives in a traditionally male role - she's a factory worker, the mains source of income for her (extended) family, and she brings Stanley into the public realm, traditionally off-limits to women. By teaching him to read and write, she gives him access to the world of knowledge, also traditionally gendered male.
Literacy here is used as a metaphor for the (traditionally masculine) public realm and the systems of circulation (monetary, vehicular, cultural) that enable participation in the public realm. Without this access, Stanley is feminized - the jobs open to him are cooking and cleaning. He is excluded from all regular circulations, unable to participate in the monetary (can't open a bank account), in the vehicular (can't get a driver's license, can't ride the bus), and in the social (he asks if he exists if he can't write his name).
After learning to read, he grabs books on auto repair, farming, and spirituality (the Bible). The Word of God is therefore relativized, placed on the same value plane as how-to books. In fact, organized religion in general is only very occasionally present - the Bible also appears on a dresser as the camera pans to find Stanley and Iris having sex. It is, however, acknowledged as a moral force - Iris, clearly a character devoted to living a "good" life, mentions at the beginning of the film that her rosary was among the objects lost in a purse snatching.
Once able to read, he enters the system and lands a managerial position with a health care plan, a car, and a house, taking his place at the head of the family, the breadwinner. Presumably, he's an industrial designer, dreaming up products that will require others enduring the drudgery of the assembly line to produce. This ending, probably the only bit of conventional Hollywood in the film, is so incongruous with all that has come before that I at least wonder if it wasn't forced in by some Studio exec suddenly worried about the lack of a feel-good ending and its potential effect on the bottom line.
Now that, according to the pundits, we've comfortably moved on to post-industrial capitalism, the film also has a slightly nostalgic feel, as though we needed the historical distance to really analyze what happened during that period.
Nevertheless, it's highly recommended - at least if you want to exercise your brain. Disregard the ending, and it's close to a perfect 10.

I really liked ZB1. Really, I did. I have no problem with extremely low-budget movies, and I have enjoyed movies with worse production values than ZB3 (if you can imagine such a thing. check out 'wiseguys vs. zombies,' if you're interested). Indeed, I prefer lower budget zombie films, because I am suspicious that Hollywood directors do not understand what zombies are 'about.'
But ZB3 was just so bad. It was retarded. I don't want to bother being dignified in my criticism. I want my 90 minutes back, etc. Except that it really only took ~80 minutes, because partway through I put it into 1.4X fast forward.
Okay, here's some criticism.
1. The pacing was TERRIBLE. Everyone talked in monologues. Even when someone just had a single line, the camera work and the editing and the insertion of a bunch of F-bombs into every sentence made the line FEEL like a monologue. At first I was excited about the 90 minute running time compared to ZB1's 70 minutes, but there were actually fewer 'events' in ZB3. It's all talking.
2. The gore effects got stupider. Just glop rubbed around on people's tummies.
3. Despite the epic exposition, there really wasn't a plot. And the exposition is indeed epic! I won't spoil it, if you're going to watch it. (Don't watch it.) But then, it's just a bunch of lame characters walking around and bickering for ~80 minutes. or fewer, if you so choose.