Branch plant sells in different quantities

We have a scenario where the client allows sales of any quantity out of one branch, but the other branch only allows sales of the same item in quantities of 3.  So, it's the same item but, depending on the branch, there is a minimum purchase of 3. 

Is there a way to handle this without setting up a new item number?  We had thought about using a new item number, but decided that it would complicate things in the warehouse.

Any suggestions would be most appreciated.