I'd like to create an active promotion for the purchase of 3 products. Actually, I've programmed the promotion as follows:

- each box entitles you to 20 promo points per unit paid for.

- the promotion is active with at least 2 items purchased (in which case the discount is smaller)

- from 3 references

The reward is as follows

- 10% discount for 40 points (2 items) 

- 15% discount in exchange for 60 points (3 ref.)

Problem: some customers order 3 boxes of the same product and are therefore entitled to the discount (3 boxes * 20 points = 60 points = 15% discount).

Except that the eligibility condition is not complete: it should be specified that the 3 references must be ordered. How can I programme this?