Hi LindStuart,
You can achieve that without an app. Follow below steps
1)create a collection and add product to created collection.
2)Create a discount “amount off” ad set it to auto discount.
3)select discount value to percentage and add 20%.
4)now apply it to specific collection and select here newly created collection.
5)Now in "minimum purchase requirement " set it to “Minimum quantity of items” and select there 3.
save it.