Do you think the events of X3 will be mentioned in Dark Phoenix since Xavier knows about them?

Ultimately, official canon is much smaller than the people who throw the term around like to think it is. Canonicity is limited to that which has actually been described in the source material. Especially in groups of writers, it boils down to what the writers specifically need to worry about for the purposes of the ongoing plot.

Fanon, also known as the generally accepted term ‘headcanon’ among fans (though 'headcanon’ also refers to personal fanon instead of widely recognized ideas), fanfic writers and Roleplay crowds, is the set of theories based on that material which, although they generally seem to be the “obvious” or “only” interpretation of canonical fact, are not actually part of the canon. Occasionally, the explanation seems good enough to just be “common sense”. The salient point to remember is that when someone shouts, “That episode was terrible because it violates the canon!”, they are very often totally incorrect.

Fanon fills in holes that the writers may have deliberately left in order to have fodder for later stories. In addition to arising from a point of vagueness in the canon, fanon can come into existence as a fact gained from a popular but non-canonical source, or taken from a different adaptation. Because many fans mistake their own fanon for actual canon, they tend to get riled up when a new fact is introduced which does not literally contradict anything canonical, but invalidates what were formerly the most obvious assumptions. Many examples of Retcon and Continuity Drift that are imagined to be violations of canon really only explicitly contradict fanon.

We talk, maybe too much, in the movie about this notion of time travel, or time continuity, where the river of time keeps going in the same direction, but you can shift it a little bit. You can put a pebble in and see at least a splash. So at the end of this movie, we want to feel like the current is going in the same direction. Charles went off and started the X-Men, but the way he did it and the people who are part of the X-Men are not exactly the same.

Again, fucking with the reboot idea. I get to take control and reboot my own movie. I rebooted the universe so now anything can happen. So here’s the plan, in my head, again giving away the plan because of the alcohol- The soda water. What happens when you use Days of Future Past to erase movies like X1, 2 and 3, yes you can erase those events that occurred, but I also was very adamant about having what we call ‘The Tivo Scene.’ The scene in that room with all the video cameras in Days of Future Past, I call it the Tivo scene. 'I developed this piece of technology that records television.’ The point is time’s immutability. The idea that time is like a river. You can splash it and mess it up and throw rocks in it and shatter it but it eventually kind of coalesces and this is, again, theories of quantum physics. It’s all based in quantum physics.

So what I’m doing with these in-betweenqueels is playing with time’s immutability and the prequel concept, meaning that yes we erased those storylines and anything can happen. That means the audience goes into the movie thinking that anything can happen. I mean anything, anyone could die. Any possibility could occur, but characters are still moving towards their immutable place. Jean and Scott, are they meant to be together? Is Scott this guy who hates schools, who ahtes authority, destined to become a leader? You don’t know. Is Jean ever going ot disover the full potential of her power? You don’t know, but we move in those direction character wise but then we have the freedom story wise to do whatever the fuck we want because we erased those three movies.