If you don't sign subject 3, it won't affect the uploading of test scores, but you can't make an appointment for subject 4, just sign it.
Subject 3 generally refers to the road driving skills test, including 16 items. After the completion of the subject three exam, you need to sign to confirm the results, mainly to prevent any unexpected situations, such as mistakes in the results and cheating. If you forget to sign your name, you can contact the test center to make it up after the exam.