If one missed Mincha and Mussaf, should one say three Maariv Amida prayers?
No, he will only say two. This is because Mussaf does not have a tashlumin, only Shacharis, Mincha and Maariv. The reason being that Mussaf is for the Korban Mussaf which can not be made up, whereas the other tefios are for the Korban Tomid which is brought a number of times a day. When davening, itis important to remember that the first tefiloh is for what you have to daven now,(in your case Maariv), and the second tefiloh is for what you are making up- Mincha.