Is it permissible to pray the Shaf' and Witr as three continuous rak'ahs, similar to the Maghrib prayer, and if I intend to separate them, is it valid for me to make the taslim after two rak'ahs and then pray the Witr rak'ah separately?

If a follower (ma'mum) intends to pray the two rak'ahs of Shaf' (even prayer) behind an Imam who intends to pray Shaf' and Witr (odd prayer), the follower has two options: either to form the intention of adding (rak'ahs) in his heart and continue following the Imam, or to sit when the Imam stands for the third rak'ah, intend to separate, recite the tashahhud, and say the salām.