Weather extremes. Blizzards, deserts, hurricanes, life rafts... If you're stranded on a cool summer afternoon, obviously you can be outside for more than 3 hours, but without shade you might get a sunburn.
It's meant as a survivalist priority list. Build a shelter, find water, make fire if you can. Food can wait until tomorrow or the next day.
I've seen that before and the 3 hours without shelter always cracks me up. Someone really needs to come up with a suitable replacement for the 3 hours part.