This tweak is not a one line tweak so you need to understand PHP coding before you can do it.
Here is what is involved in this tweak. When a product is added to the cart you make it so it adds the minimum quantity (e.g. 3) to the cart by default. Then you need to make sure that someone doesn’t change the quantity back to 1. So when they change the quantity you only let them change it if the value is greater than the minimum order quantity value otherwise you show them an error saying “You must order the minimum quantity”