Before being sent to Auradon, Jay barks like a dog to scare Carlos. Booboo Stewart portrayed one of the werewolves in the Twilight film saga.
This also happens in one episode of the Wicked World spinoff: after Mal and Evie mistakenly cast a spell that makes everyone act and bark like dogs — except Carlos, oddly enough, Carlos decides to amuse himself by having an entranced Jay to do dog tricks and taking pictures of him doing them before reversing the spell.
Anna Cathcart, the actress who played Drizella Tremaine's daughter Dizzy, also portrayed a young Drizella in Once Upon a Time.
The Character Died with Him: Not outright stated, but it's heavily implied in the Royal Wedding animated special that Carlos died at some point since the end of the third movie.
Dawson Casting: The teenaged cast is played by actors ranging from 19 to 22. Cameron Boyce was the youngest in the cast at 16 years old, which helps with making the 14 year old Carlos seem like the baby of the group.
Edited for Syndication: The first film ends with Video Credits set to Shawn Mendes' "Believe"; in later airings of the film on Disney Channel, this was removed and the beginning of the credits were moved to the beginning of the film.
Fake Nationality: Arabian Jay is played by Native American (among several ethnicities) Booboo Stewart.
This is Disney's answer to the Monster High and Ever After High multimedia franchises, which deal with the offspring of mythical/literary monsters and fairy tale characters respectively. Descendents ended up overshadowing the two to the point that both franchises were killed by the end of 2018.
This series is also another attempt by Disney to ride on the coattails of High School Musical, and arguably their most successful one to date. It helps that the two trilogies share a director.
I Knew It!: Descendants 3 confirms that popular fan theory/joke that Hades and Maleficent were a couple and that Mal is their daughter and Audrey is indeed the villain of the film.
In Memoriam: Descendants 3 is dedicated to Cameron Boyce, Carlos' actor who died of epileptic seizure on July 6, 2019. A dedication was added before the film began and the end featured various footages of him from both Disney projects and his personal home life.
Missing Trailer Scene: The beginning of the "Good to Be Bad" music video features an advertisement for VK Day as the residents of the Isle of the Lost get their applications ready. That scene was not present in the movie itself.
Fans of The Blue Fairy from Once Upon A Time will recognize Keegan Connor Tracy as Belle in Descendants. Apparently before getting the role of Belle, she auditioned for Maleficent, the dark fairy, instead. Fans of her character in Once Upon A Time have long speculated her as being a secretly evil master manipulator fairy in disguise. Imagine the rejoicing if she'd gotten the part.
The film was originally not intended to be a musical, with the songs being added after Kenny Ortega officially took over as director.
There was originally going to be a character named Aziz, son of Aladdin and Jasmine but was cut out for unknown reasons. It seems Jordan takes his place in Wicked World due to being the daughter of Genie.
This Aziz character is also a bit of casting gag. The actor who played him also played in the spinoff of Once upon a time, "Once Upon A Time In Wonderland", another Disney live action shared universe show, where he played the brother of (presumably) Aladdin's genie, who was the boyfriend of Alice from Alice in Wonderland, who were fighting against the likes of Jafar. Perhaps his scenes were cut to not have people be confused to whether the television series and Disney film were apart of the same world. Belle's actress was also in the show as the blue fairy.
It's pretty clear from the staging of Maleficent's big song around statues of the other villains that never do anything, that it was originally going to have all four of them involved, but then powerhouse singer Kristin Chenoweth was cast, which demanded she get a number of her own.