If you are writing an essay on the symbolism of the mockingbird, then you need to state that in your thesis. After you state this, go into what you think the mockingbird represents.
Both Atticus and Miss Maudie say that the mockingbird only does good things for people, that is, sing. In other words, the mockingbird is a symbol of goodness. This is why it is a sin to shoot one.
Your body paragraphs are fine (Boo Radley, Tom Robinson, and Dolphus Raymond). In each paragraph you should go into why you think these men are mockingbirds. Give examples to back up your points. There should be plenty of things you can say.
As to whether you need to state what Lee says in each paragraph, you don't. Just show why they are innocent and do not deserve to be harmed.