All three are commonly used, but there's a difference. You'd usually ask #1 or #2 right after the exam, before the student gets a grade. The third question can only be asked after the student finds out his or her grade. The answers may be different: the first or second question, right after the test, may get a reply of "It was easy," but the third, a few days later, may get "I thought it was easy, but I was sure wrong! I did very poorly."
I'm glad pob14 pointed you at the forum, rules, too. The English forum rules include both the general WR Forum rules, which you read, and the specific rules for this forum, which you now know about. They're really not so bad!