Calculate rebate in a web shop

My 2 brain cells have difficulties with this problem.

I’m creating a web shop where each item has a price. If you buy 3 you
get a lower price for each item and then even lower when you buy 6.

Each item in the product table has a unique product id and three columns
for prices; price, price3 and price6.

How should I do this? I guess one way would just to count each and see
if it’s dividable by 3 or 6 and then the magic comes … but not sure
this is the right way. Any ideas? I’m I on the right path?