Lightning kills more people in the U.S. each year than tornadoes do. Many of the deaths and injuries could have been avoided if people followed certain basic lightning safety tips. I've discussed some before, but it's well to repeat them.
When is lightning too close for you to be outside? You can estimate the distance of lightning by noting the number of seconds between the lightning and its thunder and dividing this by five to get the distance in miles. Certainly lightning three miles away is getting too close for safety. In fact, you should really play it much safer, because there is no guarantee that the next bolt will not come from another part of the cloud and be right on you. If far from shelter initially, remember that thunderstorms can occasionally advance at speeds of 50 mph or faster.
The worst situation of all is to be the highest object in an area. A boat or swimmer on the water or a person standing in a large field or out on the open plains is in grave danger of being struck by lightning. If you can't escape and find yourself in this situation, get down as low as possible — even if you feel your hair stand on end, you may avert the strike if you throw yourself down. Water is worst of all because it can conduct electricity so well.