We've done it multiple times without a problem. We've only ever tried to do it at our resort (Contempo Cafe, Pop food court and ASMo food court.) One of the times we did it at Pop, the cashier said we had to get 1 drink and 2 snacks, however every other time we've done it, we've always been able to get 3 snacks (drinks and/or food, however we choose.) Having said this, we have always done this when free dining was going on, not sure if it makes a difference. We did this in Sept. 2010, Sept. 09, and Sept. 08. We've also exchange 1 TS for 1 CS when we've needed to without a problem as well. I know it's not financially smart to do, but sometimes we've just needed the convenience.
Last year at Contempo. cafe, the cashiers were basically advertising the 1 CS for 3 snacks thing. They told us about it multiple times, even when we were just getting regular meals.