What is the ruling on praying behind an Imam who forgot the first Tashahhud and then performed it in the third رکعة? And if the prayer must be repeated, should it be repeated in congregation?

The Imam should have completed his prayer and performed the prostration of forgetfulness (Sujud as-Sahw) if he forgot the first Tashahhud and stood up for the third rak'ah. The congregants (Ma'moomīn) should have followed him. If the Imam recited the Tashahhud in the third rak'ah, knowing it was forbidden, his prayer and the prayer of those who followed him, knowing it was forbidden, would be null and void. In that situation, the congregants should have glorified Allah (said "Subhanallah").

In summary, if the Imam knew that reciting the Tashahhud in the third rak'ah was forbidden and did so, his prayer and the prayer of those who followed him, knowing it was forbidden, would be null and void. However, if they were all ignorant of the prohibition, their prayer would be valid according to schools other than the Maliki school. According to the Maliki school, ignorance is like intentionality regarding additions (to the prayer).

Some Maliki scholars prefer the view that the prayer of the congregant is not invalidated if he followed his Imam in an addition out of ignorance, but it is more cautious to repeat it.