If a mockingbird is a figure that does no harm to anyone, then we can define each of these characters as innocent victims of a small society, condemned popularly yet guilty of nothing. Mockingbirds also bring goodness into people's lives, according to Atticus.
Each of these characters brings something to his community. Tom helps Mayella with her chores. Boo saves Jem and Scout in the end. Atticus provides a model of upstanding moral behavior.
Tom Robinson actually is killed. He is shot trying to get over the fence after losing his trial. Boo Radley is symbolically murdered. He's a figurative ghost, because he is forced to remain in his house hidden away from the world.
In some respects all three have become imprisoned in the perspective of the town. Boo Radley has become the eccentric recluse, suspected of bizarre behavior and mocked if in public, Tom Robinson has become the sacrificial lamb to the "usual disease of Maycomb," who dies in despair of any justice, and Atticus Finch has also become a sacrificial lamb to justice and as a reminder of the towns prejudicial opinions.
According to Atticus, it's "a sin to kill a mockingbird." The mockingbird's innocence is symbolized in other TKAM characters, particularly the children in the story. Tom is one of the adult human mockingbirds--an innocent man who is accused of a crime he does not commit, eventually resulting in his death. Boo has been harshly punished for his youthful crimes, and then he is blamed for "any stealthy small crimes committed in Maycomb." He is relegated to spending his life as a recluse within his own home--a metaphorical death sentence that eliminates any kind of normal life for Boo. As for Atticus, by taking on the defense of Tom Robinson, he has opened himself up to all kinds of danger for both him and his family. Atticus survives the lynch mob, and his children survive Bob Ewell's murderous attack, but he is still a good and innocent man whose reputation--and his positive outlook on humanity--will be tarnished somewhat by the events of the trial and its aftermath.