How to shut off water valve to outside faucet?
In colder climates where freezing is common, there is typically a valve to shut off outside faucets just before the plumbing exits the warm interior.  If there is not a valve in that location, you'll have to trace the plumbing back and look for a valve along the way.

It's impossible to tell where the 3 branches go, without actually being on location. You'll just have to look around, or hire somebody to do it for you.  

It's defiantly possible to install your own valve, and while the main is already shut off is a perfect time. As Fiasco Labs has mentioned, quarter-turn valves are much nicer than gate valves.