I've been playing Minecraft 1.7.10 and was told that there are three strongholds, But I can only find one? How many are there?

Are you playing on your own map? If so, the buildings are strongholds and other things that pop up around the map are randomly spawned. If you get lucky you can look up that specific map seed you're using (should show the number in the details about the saved game on the main screen before you go into it and play it).
If you want to play a map that has a bunch of strongholds and you want to find them easily, I would look up
"minecraft seeds" online and a bunch should pop up. Make sure it's for the 1.7.10 update and then they should tell you where everything is!