Is it permissible to pray Witr with three rak'ahs with the intention of emulating one who prays Maghrib, knowing that the Prophet, peace and blessings be upon him, forbade praying Witr with three rak'ahs with two tashahhud, and is the prohibition here for disliking (karahah) or for حرمة (tahrim)?

There is no harm in praying Witr as three continuous rak'ahs with a single tashahhud (definition) followed by the salām. It is permissible for a Muslim to pray Witr as three rak'ahs behind someone who is praying Maghrib, because the prayer of one performing a supererogatory prayer behind someone performing an obligatory prayer is permissible by the agreement of the majority of scholars. The additional tashahhud after the second rak'ah for the one praying Witr does not cause harm, because he is following the Imam, and what is permissible in congregational prayer may not be permissible when praying alone.