Is it obligatory to observe tawarruk or iftirash in the final individual rak'ah of the three-rak'ah Witr prayer?

The basic principle for sitting during the tashahhud is iftirash (resting on the left foot). Tawarruk (sitting with the left thigh on the ground and the right foot upright) is only sunnah in the final tashahhud if the prayer has two tashahhuds. Thus, tawarruk occurs in the final tashahhud of every prayer with two tashahhuds, such as the Dhuhr, Asr, Maghrib, and Isha prayers. As for two-rak'ah prayers like Fajr and the regular Sunnah prayers, there is no tawarruk. Therefore, the worshiper performs iftirash in two rak'ahs or a single rak'ah, and also performs iftirash in the three-rak'ah Witr prayer when performed consecutively with a single taslim.