Is the prayer acceptable if the worshiper remembers, while standing for the third rak'ah, that he did not recite the tashahhud after the second rak'ah, then returns to recite the tashahhud and completes his prayer?

Your return to recite the Tashahhud before fully standing was correct, and your prayer is valid. For one who forgets the first Tashahhud and rises, it is obligatory for him to return and recite it as long as he has not fully stood, according to the saying of the Prophet, peace and blessings be upon him: "If one of you stands up after two rak'ahs and has not yet fully stood, he should sit down. But if he has fully stood, he should not sit down, but should perform two prostrations of forgetfulness (sujud al-sahw)." There is a difference of opinion as to whether one who returned before fully standing should perform sujud al-sahw or not.