A serial arsonist who has now admitted setting more than a dozen fires in the D.C. region — including one that injured several D.C. firefighters — pleaded guilty to three counts of arson in Prince George’s County and was sentenced to 15 years in prison Wednesday, authorities said.

The plea and sentence were largely formalities, as Maurice Dews, 27, had already pleaded guilty to similar charges in D.C. and had been sentenced to a 25-year term in prison. As a part of that case, he agreed to plead guilty in Prince George’s and serve his sentences “concurrently,”meaning he will get credit toward both of them for each day he spends in jail, authorities said.

In Prince George’s, prosecutors and fire officials said, Dews admitted to setting more than a dozen fires over the past five years at vacant houses, a dumpster and other structures. He pleaded guilty in three specific cases in the Landover area — a May 1, 2007, fire at 7640 Allendale Circle; a Dec. 31, 2010, fire at 3970 Warner Avenue; and a May 2, 2011, fire at 3974 Warner Avenue, fire officials said.

At one point, fire officials said, Dews applied for membership as a volunteer firefighter in Prince George’s but was denied because he had a suspended license.

Prosecutors said that while Dews generally targeted unoccupied structures, his crimes were not victimless. In D.C., authorities have said, five firefighters were injured battling a blaze Dews started at a vacant home in April. Authorities have said Dews would sometimes call 911 to report fires he had set, and emergency responders would see him at scenes, watching the buildings burn.