what if MX is making only Terra and Aqua take the exam because he wants Ven's heart for Vanitas? Maybe this whole thing of Ven not taking the Master Qualification Exam because he was the last to become an apprentice is MX's excuse to seperate Ven from he other 2? Vanitas is obviously a Ven replica enhanced with dark powers and MX wants to give Ven's heart to Vanitas so Vanitas could be a whole person whose also suuper strong with dark powers.MX might even succeed this at one point because of the scene where Terra tells MX:
"What did you do to Ven's heart!?"
"What did you do to Ven's heart!?"