Complex sentence: As of the 2010 census, its population was 3,010,232, making it the third most populous county in California, behind Los Angeles County and San Diego County, and the second most populous in the Greater Los Angeles Area, after Los Angeles County.
Split into simpler sentences: As of the 2010 census, its population was 3,010,232, making it the third most populous county in California, behind Los Angeles County and San Diego County. It's also the second most populous county in the Greater Los Angeles Area, after Los Angeles County.