Is it permissible to pray Witr three continuous rak'ahs with a single tashahhud and salam only in the third rak'ah?

What the Imam did is permissible, for it is permissible to pray Witr with three continuous rak'ahs without sitting except at the very end. By this, it does not resemble the Maghrib prayer. Witr is a specific prayer that requires a separate intention. Therefore, if the congregants did not intend to pray Witr from the beginning of the prayer, they should not follow the Imam in the third rak'ah.