John received a 3-year sentence.

• John received a 3-year sentence. (約翰獲判 3 年徒刑) (正)
• John received a 3-year-sentence. (誤)

解說：複合形容詞中的字通常用連字號連接起來，但複合形容詞與其所修飾的名詞之間不能有連字號。