Accept purchase of defined quantities only like 3,6,9 in prestashop

hi I have this problem to solve I purchased the module Minimal product quantity but I do not understand how to configure so that customers who buy the products can order 3 pieces or multiples of 3 (eg 3, 6, 9, etc) on all products. Let me explain my need is to give the user the possibility of buying any product in the catalog even a single product from various categories, but the order in the cart can not be less than 3 products or multiples of 3. Example the customer can buy 1 bottle of wine, 1 bottle of oil, 1 bottle of vinegar then a total of 3 products, but can not buy 2 products or 4 products. I'm waiting for your reply. thank you

Minimal quantity and accept multiplied quantities only

Hello

"minimal product quantity" is a module to define the minimal allowed quantity that customer can purchase. It does not define "maximum" allowed quantity, just minimal quantities.

In context of your doubts: 

I do not understand how to configure so that customers who buy the products can order 3 pieces or multiples of 3 (eg 3, 6, 9, etc) on all products

Each product page in shop has a section, where you can configure the module "minimal product quantity": https://i.imgur.com/0DjtHPR.png
Just open product edit page and set minimal allowed quantity(ies): 

Then select option to "accept" only multiplied values: 

If you do not want to do this one by one for each product, you can set these settings "in bulk", directly from module configuration page.

Go to modules > modules manager, search there for module "minimal product quantity" and configure the module: 

, or do this quickly with this form: 

So, based on these informations you will be able to set:
- minimal allowed quantity of product that customer need to purchase

- set option to accept only "multiplied" quantities like 3,6,9,12,15 etc.