3: He is the most intelligent student in our class.
   Correction: The word "most" should be preceded by "the" to indicate that he is the most intelligent student among all the students in the class. Also, "out" should be corrected to "our" to show possession.

4: Your new car is cheaper than John's.
   Correction: The word "more" should be removed because "cheaper" already indicates a comparison. Additionally, "John is" should be changed to "John's" to show possession of John's car.

5: He left college when he was 17.
   Correction: The verb tense should be changed from "he is" to "he was" to match the past tense of leaving college at the age of 17.