Without knowing what you have already written about Atticus Finch of Harper Lee's To Kill a Mockingbird, it is impossible to provide you with any specific instructions on what to put in your conclusion. However, as you probably know, the conclusion of any paragraph or essay is essentially a "wrapping up" or summarizing of the main points of what has already been written. If you have written an essay, you want to restate your thesis, drawing to a conclusive end the general statement and opinions upon which you have based your essay; then, you want to add a "clincher," or a provocative thought that extends the general statement.
For instance, since every essay is essentially an argument for the writer's points, you may wish to begin your conclusion something like this (I am guessing at what your three opinions may have been for the purpose of example here):
Because Atticus Finch displays integrity in every episode of Harper Lee's narrative, teaching his children ----------[insert a main point that you have supported], providing an example of character for them throughout the novel by --------------, displaying forthright conduct as he ------------------, he is truly a worthy and admirable character. Indeed, Atticus Finch serves well as a prototype for any man. [clincher]