To truely find out, you'll have to do some testing.

Unfortunately, there are a LOT of possibilities for such a plumbing configuration. I'll do my best to give you some ideas, but it's not going to be all inclusive:

Since they all seem to come off the cold water side, here are my assumptions:

One valve is the main isolation valve (shuts off everything)

Another goes to just the water heater (allowing you to still have water in the rest of the house while still being able to work on water heater)

The last probably goes to an outside hydrant or acts as a system drain valve.

It'll take a bit of experimentation, but you should be able to figure it out.

Sorry I couldn't help more.