Once Three Dog is dead, you're stuck with Margaret.
She also never appears in person, and therefore cannot be killed. She is not a charismatic person and has very little to say, seeming to not enjoy her new announcing duties. Many players of the evil persuasion know that you can kill Three Dog and he will be replaced by the technician Margaret. The most diverse and important station is Galaxy News Radio. Fallout 3 contains several in-game radio stations.