Complex sentence: The Buick LaCrosse is a large/mid-size luxury car manufactured and marketed by Buick since 2004, and now in its third generation, in which it is slotted above the Buick Regal as the brand's flagship.
Split into simpler sentences: The Buick LaCrosse is a large/mid-size luxury car manufactured and marketed by Buick since 2004. The LaCrosse is now in its third generation, slotted above the Buick Regal as the brand's flagship.