how to do point 4 please help me

I want to check that on opportunity if amount is greater than 10% of expected revenue then checked the coupon 1 and if 20% then checked coupon 2 and if 30% then checked coupon 3.

Coupon 1 and 2 and 3 these fields are discount coupon and this object are child object of opportunity 

How to write code for this scenario

I want to check that on opportunity if amount is greater than 10% of expected revenue then checked the coupon 1 and if 20% then checked coupon 2 and if 30% then checked coupon 3.

Coupon 1 and 2 and 3 these fields are discount coupon and this object are child object of opportunity 

How to write code for this scenario