SAUGUS – The third suspect in the Riverside Cemetery thefts has turned himself into police and is being held on $5,000 bail.Dennis S. Stack, 35 of 200 Somerset Ave., Winthrop was taken into custody as he stepped off a plane at Logan Airport Wednesday evening.Stack, along with Craig P. Nolan and Joshua Shulman, both also of Winthrop, allegedly stole 680 aluminum flag holders from veterans’ graves on Oct. 7 and later sold them to a scrap metal dealer.Nolan and Schulman were charged with the theft Monday but Stack had fled to Florida.Lt. Leonard Campanello said Stack called the department earlier this week and offered to turn himself in.”He called us and said he was getting on a plane,” Campanello said. “His brother had called him too.”Campanello said he thought it was the media attention coupled with the fact that he probably had already spent the money made from selling the holders that prompted Stack to call.”And I think he saw the cards were stacked against him,” he added. “He hasn’t given us any trouble.”Stack will also have to answer for an outstanding warrant for possession of a Class B substance and a failure to appear in court.Nolan and Schulman have not been arraigned yet but Campanello said he expects they will be since a summons for each was filed earlier Thursday.Nolan and Schulman are being charged with accessory after the fact, larceny over $250,000 and defacing gravestones by removal of veterans’ markers.Stack is being charged with larceny over $250,000 and 680 counts of defacing gravestones by removal of veterans’ markers.