Are the confirmed Sunnah prayers obligatory for the prescribed prayers (Fajr, Maghrib, and Witr), and is one sinful for abandoning them?

Of the category of prayer, only the five daily prayers are obligatory, based on the Prophet's (peace be upon him) statement: "Five prayers are prescribed in a day and a night." And when he was asked, "Is anything else obligatory upon me?" he replied, "No, unless you perform supererogatory ones." Therefore, anything in addition to the five daily prayers is considered supererogatory. There is no sin upon one who abstains from voluntary prayers, but by doing so, they miss out on much good.