Dstuva makes an excellent point. When students are limited to or limit themselves to three main points their writing is sometime weakened due to the restraints. The question I think your asking is what thesis might enable you to address the points you mentioned, and I think a thesis that addresses author style with regard to the: literature quoted, the dialogue between characters, and the character speech and dialect.

I'll start the discussion off by just telling you that professionally written essays do not have any kind of requirement to limit as essay to three supporting areas, and therefore, student essays should not, either.  If you want to present more than three elements, do so.  The limit of three supports is artificial, and need not be followed (unless, of course, your teacher requires you to).