Hello Mark. I'm Joelma from Brazil. So these three words are very similar and are synonyms. You are right about the meanings (assunto - subject, tópico - topic, tema - theme). So, in general, these words indicate what is being talked about. E.g. Qual o assunto do seu livro? Quais tópicos seu livro aborda? Qual é o tema do livro? I hope that was useful. :)