The answer to your question would be that the sentence that contains three common nouns is the following one: She has never upset an object or as much as brushed a magazine onto the floor. That is, the correct option would be C.
The three common nouns found in the sentence in question are the following ones: object, magazine and floor. A common noun is the generic name for a person, place, or thing in a class or group. As opposed to proper nouns, they are not capitalized.