The exchange of 1 QS for 3 snacks has never been an official use of the dining plans, but was widely done when Disney put the "exchange dessert/drink entitlement for a snack" into the brochure. Since the QS meal went to just entrée and drink AND the exchange wording disappeared from the brochure, we've seen fewer reports of being able to do the 1-3 exchange within the parks. It's still common at the resorts, but not the parks. I don't remember seeing even one report of someone doing so at the Flower & Garden booths. That doesn't mean it didn't happen, but I think it does mean it is not a given as it was for a couple years.
By all means, I would ask at EVERY booth I wished to do so. One tip from the past, ask only for your 3 snacks as a QS credit in their own transaction. Don't try to add drinks or extra snacks that you're paying OOP for into the request. That always worked better when it was common to do so.