The average price of a gallon of self-serve regular gasoline in Riverside County dropped eight-tenths of a cent Sunday to $6.082, its third consecutive slight decrease after a 16-day streak of increases that saw it rise 34.1 cents. The average price is 7 cents more than one week ago, 22.5 cents more than one month ago and $1.436 more than one year ago, according to figures from the AAA and Oil Price Information Service. It has risen $1.531 since…