Whether it's canon or not is sort of subjective I feel... I mean, it's not like any of the creators have ever come out and said it wasn't, plus the fact that they referred to one of the characters again (Wallace) in a CD drama later feels that it wasn't meant to be just a one-time bizarro world happening.
The creators don’t need to come out and say its non-canon.
1. Wallace simply doesn’t exist in the tv series. He’s in one audio drama but those tend to be just for fun or not too concerned about continuity. All that means is that audio drama takes place in whatever timeline movie 3 takes place in. There is no mention of Wallace (outside the dub which tries so hard to fit in with the main timeline) even when Daisuke travels to his home state of New York in the Christmas World Tour campaign.
2. Assuming this movie takes place around its release date (between episode 14 and 15) then Daisuke and company should know what the golden digimental is in episode 20. If it takes place between episode 21 and 22 then Daisuke should know what the golden digimental is in the movie (he says “these are ours?” But we are to assume, if movie 3 is canon and between episode 21 and 22 he’s just recently used it). Both times are treated as the first time. At any rate Quinlongmon in his exposition dump episode only bothers to explain the one in the series and never thinks to explain this holy phenomenon.
3. Angemon and Angewomon know they can evolve to Ultimate. And no Cherubimon’s lonely heart warping reality and causing time to go back would not make them stronger. That has always been a terrible fanwank of the highest order.
What gets me is just that there was really no plausible explanation for how they were able to evolve to Seraphimon and Holydramon out of the blue
The explanation of course is that the movie wasn’t beholden to what the tv series did. The movie was its own independent side story roughly based on what was going on in the show at the time (kids on summer vacation, new kids can only armor evolve, Takeru and Hikari had access to their Digimon evolutions outside dark tower area) Angemon and Angewomon could probably evolve to Ultimate in the movie because at the time nothing said they couldn’t, the series was focused on armor evolutions at the time.
Movie 3 has its own plot holes (the writers seem to have no clue how far Colorado is from New York, Wallace keeps calling Terriermon the wrong name, this American kid keeps speaking Japanese to his American mother) but anything it contradicts or the series later contradicts is just a byproduct of the movie being made as an independent standalone story that wasn’t bogged by what the show was doing or would do.