Is what happened considered a Ghamus oath, and does fasting for three days expiate this sin?

Your false oath that you did not tell your mother falls under the category of a "Ghamus Oath" (a sinful, false oath), and you are obligated to perform sincere repentance. Scholars have differed regarding the Ghamus Oath: whether expiation (kaffarah) is required for it or not. Abu Hanifa, Malik, and Ahmad, in one of his narrations, held that there is no expiation for it due to its gravity. In contrast, Al-Shafi'i and Ahmad, in another narration, maintained that expiation is obligatory. If you wish to perform expiation, the expiation for an oath is, in order: feeding ten needy persons, or clothing them, or freeing a slave. If you are unable to do any of these, then fasting for three days.