Can you help me with zoho CRM and the discounts ?

I have some special discounts we need to justify. 

For example, I need a -10% for a new client, a -5% for 2 buys, a -7% for 3 buys...

How can we set this kind of discounts ? 

I'd like to have the lists of discounts in wich we can pick the correct one. Of course, total has to be updated with the new amount.

Thanks for any help.