What should I do with 1 of 3 donuts sold accidentally for the price of one?

I went to buy two donuts and a drink at a Dunkin' Donuts drive-thru. The clark at the window gave me the donuts in a paper bag and then handed me my drink and receipt. Than I drove home. On the way I started to eat my donuts. When I reached for the second one, I noticed a third one. I thought, no big deal. But I was to full to eat the third one anyway. When I got home, I looked at my receipt. It showed that I only purchased one donut and a drink! What should I do?