For an RPG to become canon for a game world would be a bit of a stretch. There are many possible outcomes for BG3 so which one is the canon outcome? Also for canon what class if the players character known as for the canon story? Could the "events" become canon, sure, that is easy. The City of Baldurs Gate was attacked by the plot involving the Dead Three attempting to control an Illithad Elder Brain. A band of Adventurers where able to defeat this plot, but not before "hundreds" of people where killed. The City however survived and has begun the process of rebuilding.
In this way the game could be canon and give an open legend that a DM could add to his game world. As for the various "companions" their contributions could be vaguely noted or their participation hinted at without specifics.
I think it's possible to retain the origin characters as "legends" who had a pivotal role in the story, and then the DM can decide what part that was. That way you can have the best of both worlds.
