She will have been working vs she will be working

By ten o'clock , she will have been typing for three hours

At ten o'clock, she will have three hours of typing work behind her.

By ten o'clock , she will be typing for three hours

No correct sentence because Future Continuous tells about what happens exactly at the referenced time in the future. It cannot be used to tell about actions which have started before the referenced time in the future and last until the referenced time.