I was confused about the 3 hours without shelter bit, as it seems most people would be totally fine without shelter until around the 18 hour mark. But there is some important context:

- You can survive three minutes without breathable air (unconsciousness), or in icy water.

- You can survive three hours in a harsh environment (extreme heat or cold).

- You can survive three days without drinkable water.

- You can survive three weeks without food

Interesting, I thought water was more important than shelter. But sources cite the same.