Sylvester Stallone began working on the Rocky III (1982) script immediately after completing Rocky II, with the intention of the series being a trilogy. Originally, he had no plans to make a fourth film.
Sylvester Stallone himself wrote the paperback novelization for this movie. The novel is mostly in first person, from Rocky's point of view, written in the same choppy English in which Rocky speaks. Scenes in which Rocky is not present (such as Apollo Creed consulting his associates, or Paulie alone with Adrian) are in standard third-person, in proper English.
Analysis by Philadelphia locals tracked the route Rocky took through the city during his training run when all the children ended up running with him. If he took this actual route from his South Philly house to the top of the Art Museum steps he would've run approximately 30.2 miles in one day - 4 miles more than a marathon.
During his preparation for the film, Sylvester Stallone was bench-pressing 220 pounds, when the weight fell and tore his right pectoral muscle. This was shortly before the fight scene was to be filmed, and ultimately, the scene was shot with Stallone still badly injured.
Originally, Adrian was supposed to be at the big fight. However, because Talia Shire was working on another movie at the time, the storyline was changed to having her stay home and watch the fight on television. The scenes of her watching the boxing match on television were shot, and then edited into the movie several months after filming on the fight scenes had finished.
As he had done with the original, Sylvester Stallone incorporated biographical elements from his own life into Rocky's story for this film. In particular, Stallone used as a central plot point the concept that yesterday's heroes are quickly forgotten. In the film, this manifests itself in terms of people quickly forgetting about Rocky's exploits in the title fight. In reality, Stallone experienced a similar sense of being quickly forgotten after his two post-Rocky (1976) movies, Paradise Alley (1978) (Stallone's directorial debut) and F.I.S.T. (1978), both of which underperformed at the box office.
In the first film, Rocky becomes angry when Gazzo's driver suggests he take Adrian to the zoo because "retards like the zoo", insisting she is just shy. In this film, Rocky proposes to Adrian at the zoo.
After the bell rings, signalling the end of the second round, Sylvester Stallone and Carl Weathers are seen pushing, shoving, taunting, and ultimately being pulled apart by their respective cornermen. They continue to taunt each other before returning to their corners. Stallone revealed later they were actually angry with each other and were not acting at that point, several blows that were supposed to miss him landed and the carefully choreographed fight, which they spent months meticulously planning out, went off-track during that scene, but he liked the reaction the scene produced. He decided to leave their momentary breaking of character in and the viewing audience never realized the two actors were in reality quite livid with each other.
Chuck Wepner, the real-life inspiration for Rocky (1976), was offered the part of a trainer named "Chink Weber". According to Wepner, he read for Sylvester Stallone but did very poorly. The character was deleted from the script. The name "Chink Weber" ended up being used for Sonny Landham's character in Stallone's movie Lock Up (1989).
In the fight scene at the end, right before Rocky is knocked down for the first time it can be heard that one of the commentators calls Rocky Rambo instead of Rocky. This was Sylvester Stallone's next famous role.
During the ambulance drive to the hospital during the opening credits, the movie poster from Paradise Alley (1978) also written, directed by and starring Sylvester Stallone can be seen on the back of a bus.
During the commercial filming scene, the clapper-board reads "Director: John Pleshette", the real name of the actor playing the director. Also, Duke (Apollo's Trainer), the Agent and the Meat Foreman (played by Tony Burton, Leonard Gaines and Frank McRae respectively) are all referred to by their real Christian names in the script, while the Referee is introduced as Lou Fillipo, again the real-life name of the actor (who is also a professional fight referee).
Before the main event the two commentators say that in order for Rocky to win he needs to draw first blood. This is a foreshadowing of his next major movie franchise Rambo, whose first movie was First Blood (1982).
The trivia item below may give away important plot points.
According to the director of the original film, John G. Avildsen, in a 1980s newspaper interview, one of the main reasons behind his not directing this sequel, besides preparing for his role as director for Saturday Night Fever (1977), from which he was eventually fired, was that he didn't approve of the story. Although, he was fond of Stallone's original concepts for the two films which would have made the series a trilogy. The plots would have consisted of Rocky going into politics, running for Mayor of Philadelphia on the Reform Ticket, and winning. Only to have be scandalized, when Paulie is caught stealing from the treasury. Rocky honorably takes the blame himself, and is kicked out of office, and ends up penniless just as he had at the beginning of the series. Coincidentally, this is similar to a scenario that Stallone and Avildsen collaborated on for Rocky V (1990)