The rule of threes for survival is a convenient way to remember the order of importance for each basic survival need. In extreme survival situations, you cannot survive more than:
Shelter is necessary, but it is not difficult to provide it. You should always have sunshine and wind at your disposal. This can be protective clothing on the move and outdoors, or trees, tents, or buildings when they crouch down. The time of year, climate and geography dictate which form of shelter you need.