Shortening Maghrib Salah

A: First, your Tayammum is not valid, as you did not wipe your face with your hands after striking them on the ground. Second, it is impermissible to shorten the Maghrib Prayer, as shortening is a ruling confined to four-Rak`ah Salah (Prayer consisting of four units) such as `Isha'. Third, you and your wife must repeat the Maghrib and `Isha' Prayers because your Taymmum was invalid, and because you left out a Rak`ah of the Maghrib with the intention of shortening Salah, while it cannot be shortened.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.