approving levels

Please Help

 I need to define approval levels like this:

amount         AB#            name

 

100              76800          John

2000            76805          Max

15000          77000          Lewis

But when a purchase order is created with an amount of 3000 then the system automatically send it to the first approver in this case AB# 76800 John, then to the second Max and Lewis at last.

I need that when a purchase order with an amount of 3000 the system assigns the approver #2  AB# 76805 MAX.