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There is no built in feature but yes it's possible to set a minimum quantity of a product. When you add quantity property to your product and add the option properties, you simply start the option from that minimum quantity.
I created a demo for you: http://form.jotformpro.com/form/32465804515959
In this demo, I have set the minimum qty of banana to be 3, now the moment you select banana the qty by default will be 3.
How you can do it:
1. Add payment question in your form.
2. Add a product.
3. Select the product from the list and then click "Add quantity property" (don't select custom quantity checkbox.)
4. In the "Add New Option" window, enter the required quantity. By default it will have 1 - 10 but if you want to set 3 as minimum quantity, start it only from 3 -10.
Hope this helps.