I honestly don't think anyone except hardcore fans remember a whole lot about X3. It was a bad movie that made money but left zero impressions. The Phoenix in that never had fire, never went to space, and was not even the central focus of that awful movie.
Just as there was overlap between seeing Batman vs. Joker, or Bruce Wayne romancing Selina Kyle, there will be overlap here. It is up to Simon Kinberg to deliver something significantly different and better than that. And he better since he dipped into this well a second time.
Plus, again if the first teaser features Jean Grey going the full star child, a la 2001, while floating in space, it will look quite different than anything ever done in a superhero movie. That is how you hook audiences. If it is just a beat for beat replay of X3, this movie has much bigger issues than how to market it.