A 3 year old could walk 15 mins to the shops. But they might not. There might be a snail on the path that they have to stare at for awhile and then they might want to go the other way, and then it might rain and they want to swoosh through the grass. And they might feel too tired to walk back home.

I'd have some way of carting your toddler home as a backup plan, because you really don't want to be pushing a pram with a toddler over your shoulder if it rains or if your DD cracks it.