Why would you see a flash of lightning and then 3s later you hear the thunder if you are 1km away?

This is because light travels much faster than sound. Light travels at approximately 299,792 kilometers per second, while sound travels at a much slower speed of about 343 meters per second. When you see a flash of lightning, the light reaches you almost instantaneously, but it takes some time for the sound of the thunder to travel the distance to where you are. By calculating the time it takes for the sound to travel 1km (approximately 3 seconds), you can determine how far away the lightning strike was.