Well it’s actually not called The Mummy 3, it’s actually The Mummy: Tomb of the Dragon Emperor. The following was posted by the director Rob Cohen on his production blog.
The blockbuster global Mummy franchise takes a spellbinding turn as the action shifts to Asia for the next chapter in the adventure series, The Mummy: Tomb of the Dragon Emperor. Brendan Fraser returns as explorer Rick O’Connell to combat the resurrected Han Emperor (Jet Li) in an epic that races from the catacombs of ancient China high into the frigid Himalayas. Rick is joined in this all-new adventure by son Alex (newcomer Luke Ford), wife Evelyn (Maria Bello) and her brother, Jonathan (John Hannah). And this time, the O’Connells must stop a mummy awoken from a 2,000-year-old curse who threatens to plunge the world into his merciless, unending service.
http://www.robcohenthemummy.com/synopsis.php
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I haven’t seen the first two films in ages and I have yet to see the Rock’s Scorpion movie. I’m glad that Brendan is back, but Rachel Weisz turned down returning in the film for a variety of reasons. So as you can see, Maria Bello takes Rachel’s place in this sequel.