XX should really be thrown in but it isn't a quote-unquote canon entry.
DXX is just an alternate, streamlined take on Rondo's canon for the most part, so
technically it is a canon entry when push comes to shove. If you examine most of the plots that derive from or reference the events of Richter's life, they fall into two categories: either specifically mentioning the events of Symphony of the Night, or drawing from a generalized version of Rondo's plot that could easily refer just as well to DXX.
I think the reason that was done for so long was because Rondo spent over 15 years locked out of the North American market and DXX was our only point of contact with that story, so there was a "soft requirement" to compose references that could apply to either version of the story. Ironically, even Symphony only references characters from Rondo who reprise in that game: Terra, Iris, and Annette aren't mentioned at all, and there are no specific references to any of the other bosses in a narrative sense (though several of Rondo's bosses
do appear again to confront Alucard).
Honestly, it's a good approach, and Dracula X Chronicles later took the best of both game's distinct plot elements -- or at least took Rondo and then added DXX's original elements back in.
Really, they're all canon because they're all the same story just with some different emphasis from different storytellers, and none of the differences in storytelling have made or broken any other later tales that tied into this one (so far).
Same fairy tale, different storytelling traditions.
Kind of like King Arthur, but gothier. And with pretty boys with whips.