Hi,
There are some settings that you need to enable to make this works the way you want.
To block adding amounts other than these 3 to the basket, please go to plugin main page >> Add to Cart Validation >> Validate and add notices.
The + icon takes the step (which is fixed), in your case this won’t work since you have a step of 3 then 6, to enable dropdown for this case, here is a workaround:
Go to Quantity Dropdown page (plugin settings), Enable, go to Maximum Quantity page, enable section and enable per product, then go to this particular product, where you’ve defined 3, 6, 12 as fixed, just enter 13 in the maximum field, and you will have a drop down that shows 3, 6, and 12.
Let me know if you have any other question, and if you found the plugin & support useful, please consider leaving us a rating here or on WP.org plugin page
Omar