Is a triple threat an idiom?

"Triple threat" is not technically an idiom, because it's a literal description of someone or something with three capabilities.

However, it's often used figuratively to describe a person who is exceptionally talented in three areas, like acting, singing, and dancing. In this sense, it acts like an idiom because it's understood to mean more than just the literal sum of its parts.

So, while not technically an idiom, "triple threat" can be used in a way that makes it function like an idiom.