Wa Alikum as Salaam Rashid! Take care and enjoy!
Hira, the only way you can pass unit 3, is if you answer as many papers as you can and compare them with the mark scheme. No teacher can help you with it, or maybe its just mine. She might be able to help wiht unit 1 and 2 past papers but unit 3...you're all on your own. No matter how much you study the theory, it won't help wihtout answering the past papers.
Here's an advice, answer every paper more than once, twice even three times if you have to, until you get that hang of it.