User blog comment:Jimeee/PS3 release dates for Dragonborn, Hearthfire and Dawnguard - Euro Update/@comment-91.85.57.24-20130210134345/@comment-1840877-20130211153849

As somebody said previously, Bethesda are probably doing this as a "best foot forward" sort of thing. Lots of Xbox users gave the most praise to Dragonborn, which is why it's the one getting released first.