Without more context, it's impossible to know whether using a definite article is permissible here. Sentences 1 and 3 sound like they're talking about words in general. Sentence 2 needs to refer to a specific word--that's what the definite article is for.
"Put" is a verb that asks for two objects, one direct and one indirect. You can "put a definite article before the noun", but you can't "put a definite article" without a prepositional phrase explaining where you're putting it. If you want to avoid specifying the indirect object, use the verb "use".
Oh, and "a definite article" and "the definite article" both sound fine to me in this context.