If you wish to create a coupon that applies only when a certain number of items is purchased, an option would be to create a coupon as such, e.g.

Name: 3 For 2
Coupon Section: Coupons
Coupon Code: 3FOR2
Minimum Quantity: 1
Maximum Quantity: 1000

Step Value: 3

• This will work on multiples of x (in this case: 3) since this is the step value.
• So if you have 3, the coupon will apply, but also when you have 6 and 9 and 12, etc.
• The coupon will apply if there are 3 or more items are in the basket

Discount Amount: GBP: 30
Live (checkbox): Ticked

There are other settings that can be applied, such as "Apply to all Catalogue Items".

Ideally, you would select the specific catalogue item for this.