If I were you, I wouldn't try to see the differences between them in such single sentences because they usually make sense in a context. On the other hand, a few explanations go below; A. I am helping you with your homework.
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A. I am helping you with your homework tomorrow. (exactly tomorrow)
B. I am going to help you with your homework this week.
C. Yes, I will help you with your homework. It's a promise.