How to check product quantity is multiple of 3?

Original title: `3-packs`

I would like to sell custom 3-packs. Could be anything. Color options red, white, blue, or green in any combination of 3. How can I make the form verify that they have entered quantities in multiples of 3?

Hi,

You can set your '3-pack' as a stand-alone product.

Then create a the color selection fields separately and make them 'required/mandatory'.

This form cannot be submitted before 3 colors are specified.

So that you don't need to check it's a multiple of 3 since quantity will be taken as 3 by default as long as this stand-alone product is selected.