If the boxes are identical, look at UnitOfMeasure.  Calling your UoM "3BOX" should be easy enough, or do purchasing with "3BOX" UoM, and sell by "BOX" in multiples of 3. 

Watch your prices / costs though, if you do any kind of UoM conversions.  They are set based on the StandardUoM, and the way conversions work is not always intuitive.

If the boxes are of different components, using a kit makes sense.  Maintain inventory of the 3 components independently, and explode the kit components on invoices (so inventory is picked / consumed that way).  If you have BM, there is even a disassembly option to handle breaking a kit apart into components (for sale) if you purchase / receive based on the kit of 3 boxes.

What do you do if you use serialized items in BM? We have a serialized item with 2 boxes. During shipping we always have to add an empty box which seems fudgy. Kits would be awesome if they supported serialized items but they do not. :(

Set up a kit to explode on invoices / orders (not for pre-assembly), and the components can be serialized.  Since you'll never have QoH for the parent item, and costs for an exploded kit come from the components, it really doesn't matter what the valuation is for the parent item code.

Hello Kevin. We tried this but onece the kit is added to a sales order, you cannot select or distribute a component serial number either during SO creation or in shipping data entry. Something we are missing?

Hello Kevin. We tried this but onece the kit is added to a sales order, you cannot select or distribute a component serial number either during SO creation or in shipping data entry. Something we are missing?