Given the advancements in CGI technology since the release of Initial D's First and Second Stages in 1998 and 1999, all of the flashback scenes shown in the movie were re-animated with the same high-quality animation used in the movie, as opposed to simply editing in the older, 'rougher' scenes from the TV series.
All cars belonging to new characters in Third Stage are Toyotas. The (SW20) MR2, and the Celica GT-Four. They are the first Toyotas to be used in the series that are not AE86 Sprinters (Corollas in the US).
The course in this movie, Irohazaka, is located near Nikkoh in Tochigi prefecture. It has two different roads; one going uphill and one going downhill. Irohazaka got its name because in the ancient Japanese days, it was a mountain trade route. "I, Ro, Ha" are the first three letters of the old Japanese alphabet, given as a joke to the route's many corners. Every corner is named after a Japanese letter. If you watch closely, you'll see that every corner has a blue sign with a different Japanese letter.
The items Natsuki Mogi is seen selling to a customer in the ending clips are VHS copies of Initial D: Second Stage (1999) and "Hot Version" which had on the cover, Keiichi Tsuchiya standing by his own AE86 Sprinter.
Despite the title, this is actually a feature film and is not a TV season.