Your problem is very difficult to address without serious customizations under-the-hood, which manager doesn’t allow.
However, I kind of understand where you are coming from. You need to allocate freight-in cost by location.
In that case, what I would do is expense the freight-in cost a distribute to three tracking codes: A,B and C.
Unless you have regulations that tell you how to account for inventory, I think this should do it, otherwise, you might have to do some journal entries to reallocate costs.
Manager does not have the ability to segregate average cost by location. If you want to do that, you will need to create individual inventory items for the different locations. Then, you will need to use production orders to convert an inventory item at location A into an inventory item at location B. Once you do that, you can add separate freight-in costs to the individual inventory items.