It's not necessary for the MCAT, but I don't think T3 and T4 regulate plasma calcium levels. They are primarily involved in metabolism.
You are correct about the functions of calcitonin and PTH, and also that PTH acts via a secondary messenger system. Also, keep in mind that PTH has a much more significant effect than calcitonin. Your body can regulate calcium perfectly fine without calcitonin, but not without PTH.