Triple coupons were a popular grocery store incentive in the early 1990's. Now they are not nearly as easy to find and generally are an incentive found as coupons to be clipped from the specific in-store advertisement. There is usually a limit to the number of coupons that can be tripled (4-6), and the amount on the coupon that can be tripled (usually $0.50). In order to receive a triple coupon, the newspaper coupon almost always needs to accompany the coupon to be tripled.
In spite of these rules, it is an incentive that can greatly reduce the price of a product. If the store triples a $0.50 coupon, you receive a savings of $1.50 - since many of the "low value" coupons are issued for low cost items, triple coupons often equate to a free item.
You can visit Grocery Coupon Guide