How do i find the other two strongholds?

The eye of ender locks on to whatever stronghold you're nearest to when you throw your first one. Therefore, the only way to find multiple strongholds is to get lucky while mining or use a chunk search program. Search for bookcases, and since stronghold libraries are the only places where bookcases naturally occur, this will allow you to locate all three strongholds. This is cheating though, so mining for ages is the only legitimate way to find all of them.

Try going to the 0,0 coords and heading in the oposite way as the stronghold you already found.The eyes of ender will take you to the nearest stronghold.The third one should be a little more tricky since you will have to go away from two strongholds so be sure to go away from BOTH strongholds you already know.