Discount type: Product discount
Product category: … your category …
Min matching products amount: 3
Auto coupon: yes
This will work for one free item. If you tick allow multiplication, the free product amount will be multiplied (2 if 6 matching products, 3 if 9 matching products).
If you want the customer to select different products if he has 6 or more items; there is no ‘neat’ way to achieve this. You could create a second coupon, with min matching amount set to 6, but if you have many products to choose from, there are probably too many options on screen, which will be a terrible User Experience.