You raise some interesting points and it sounds like you've mulled this over before. I guess then, that canon (if it were to be singularly defined) should be the generally accepted "shared abstraction that surrounds and incorporates" Fallout 1, 2 and 3.
Do you feel that it is canon? And if yes/no, what are your reasons?
Some thoughts below:
*Authors may contradict in-game facts with how something was meant to be or interpretted. Unless such occurrences are either fixed in a patch or addressed in a sequel release I don't believe these statements would or should be generally considered canon (depending on the magnitude of the change they had proposed).
If an author said that a certain NPC's shirt should have been blue, or that someone's dialogue was suppose to be different... these are mundane things. Whether they're considered canon doesn't really alter the universe.
What some people are proposing is that Fallout is not canon and thus an alternate Fallout universe. It is implied that at some point Fallout 3 changed history to become what it is, which doesn't appear to be the case.
It should also be considered that an author statement or interview is not going to be viewed by 99% of the people playing the game, so unless it's actually put in, it's difficult to get that correction/clarification out there. The Fallout wikia is extremely helpful in this regard however, as I have noted some lore corrections from the Fallout bible. Mind you, they have been rather minor. I've found it an excellent resource.
If ten years after the release of Fallout 11 RichBob buys the license and posts on the internet that he just bought the license and has decided the Master never existed, no one will or should give a damn.
I don't think this is really a grey area though and certainly doesn't apply to Fallout 3 as it stands currently. This would be a complete change of a major historical event, which would be creating an "alternate universe" Fallout.
It's similar to the new Star Trek movie in which Vulcan was destroyed. They've gone back to pre-kirk times and fundamentally altered a major event in that universe... which has created a new one. This is canon, but only in the way that it is a new and seperate Star Trek timeline all together. It's not canon to the existing history of the original Star Trek and it's subsequent series'.
As far as I am aware, Fallout 3 has not changed any major historical events.
I believe these are absolutely canon, because they aren't just proposed as "this is how it should have been" they have actually made the correction. Typically these aren't major corrections.
Just like the previous point, I'm not aware that Fallout 3 has retroactively changed anything
major... now, I would believe that they may have changed something minor, either inadvertently or because they felt it didn't make sense in the first place. I'd be interested in seeing an example of this actually.
Or retroactive fixation of variable outcomes:
I believe such changes can also be considered canon. If these are considered fixes/patches, they are official changes applied to the game. I'm not aware of any such occurrence that was majorly history changing (were there any?). I can see how this would get confusing if Fallout 2 had been patched and completely changed one of the side quests. This would be confusing to future generations who may or may not install the patch.
I think such complications are greatly reduced in today's generation, since most patches will prompt you to update when you load up the game (rather than having to seek it out).
I think we could get into a deep, philosophical discussion about what constitutes canon, but I'm mostly talking about how it relates to Fallout 3 and whether it is inline with Fallout 1/2 or a completely new universe/series.
When you're dealing with that much lore (from over a decade ago) it can be difficult to have the pieces of the puzzle come together perfectly. Even Fallout 1/2 are riddled with minor inconsistencies between the two of them, so it should be expected that Fallout 3 would have some.
So, are there minor differences? Yes. Are there major, history breaking/altering inconsistencies? No, I don't feel so... but I am open to discussion.
This is a stimulating conversation, haha.