。
He's the class clown.
Literal
He [topic-は] third-poster [copula-だ].
三枚目 (さんまいめ, 'third poster') — from kabuki, where the third billboard was the comic-relief role, contrasting with the 二枚目 (handsome romantic lead) and 一枚目 (the star/main billing). In modern usage, 三枚目 simply means 'comedian, funny guy, comic figure.' A great example of how kabuki theater conventions seeded everyday Japanese vocabulary.