Allow specific multiple number of items per product

[SarahB]

Hi, I wonder if anyone can help. On our site selling olive oil, each type of container comes in different sized boxes with a different number of units in each one.
What I want to do is establish a multiple number for each product:
e.g. the 5-litre plastic bottles come in boxes of three so I want to only allow multiples of 3 to be purchased (3, 6, 9, 12, etc.).
Does anyone know if this is possible and how to establish these multiple requirements?
Fingers crossed that it is possible. Thanks for your help.
Sarah.

[abantecart]

Why can't you just have a product with 15 litre?  Mention it comes in the box with 3 bottle of 5 litter each.

If you do not sell bottle separately, make a box as one product unit. Obviously, quantity multiplier will work if you have 3 in the set.

Other box sizes will have a different product set.

I think it is easier for you and customers. Less options to select.