We're talking about a few thousand rpms per minute for a pump like that without water against a dry bearing, which at that speed is going to heat up and deform the spindle and bearing pretty quickly. In 3 seconds I doubt much damage would have occurred, especially if the bearing is ceramic, but you'd have to wait for Bill to give you his opinion (backed up by fact no less!) for a definitive answer.
All I know is that my Iwaki hasn't been any less for the few occasions I've turned it on dry by mistake, nor has my aquarium pump which has been running with lousy plastic bearings for five years now.
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