The answer to your question would be that the sentence that uses the modifier "unique" correctly is the following one: The building's unique design surprised us.
An adjective can exist in three forms: positive, comparative, and superlative. The positive form is the base form of the adjective (sentence 2), the comparative form expresses a higher degree of some quality (sentence 3), and the superlative form expresses the highest degree (sentence 1).
The adjective in question, "unique", refers to an entity that is the only one of its kind. That is why it cannot have a comparative or superlative form.
