Decided the following sentence is grammatically correct or incorrect. había tres bebés en el cine.

Intuition may be that “habían” is the correct form, since there are plural babies, but the sentence is actually grammatically correct. Haber is a verb that, in this meaning, is not affected by number. In the present form, for example, you would say “hay una bicicleta” or “hay dos bicicletas”, using the same verb for both the singular and plural.