Boom

Boom is the third and youngest, reveled from Dragons: Rise of Berk. He appears in the 18th episode of Dragons: Defenders of Berk as a baby.

Information
Boom is a baby Thunderdrum that Thornado is currently taking care of. Boom's name was originally going to be Lloyd as chosen by Tuffnut but was changed by Fishlegs to make more sense. But Tuffnut still wants his name to be Lloyd. Or Darren. Like Bing and Bam, they were chased by wild dragons from Dragon Island to the sea stacks.

According to the Book of Dragons in Dragons: Rise of Berk, Boom is the youngest and most undisciplined of the triplets. And Snotlout found that out...the hard way.

Trivia

 * Boom shares his color with the wild Thunderdrum that appeared in We Are Family Part I as well as Thornado's Friend who appeared in How to Pick Your Dragon.