During the storm, the glass in three of the windows was broken.

"Broken" is an adjective form that describes the state of the windows but does not provide the necessary verb structure for the sentence. It lacks a subject-verb agreement and does not convey the action of breaking in a complete manner. Therefore, it fails to fit grammatically within the context of the sentence.

"Breaks" is the present tense form of the verb, which does not align with the past context of the sentence describing an event that occurred during the storm. This choice suggests an ongoing action rather than a completed one, making it grammatically incorrect for the intended meaning of the sentence.

"Were broken" is a correct passive construction but conflicts with the singular subject "three of the windows." The verb form "were" implies a plural subject, while the sentence's context requires a singular agreement with "glass," thus making this choice incorrect in this specific instance.