The first is fine. The second is unnatural as a stand-alone sentence. The third is OK.
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Ol how can I make the second sentence natural?You can't. "used to be teaching" cannot be found in COCA at all (a corpus of American English), and other instances of "used to be -ing" with other verbs are few and far between. It's best to avoid all those variants and stick with the plain "used to" plus a plain verb.