Setting Circular Replication in mysql
Rolando told you how to do it and his answer is excellent.  The truth is, circular replication is an awful, terrible, terrible idea.  Don't do it. Ever. "Absolutes are never true," but in this case, it's pretty close.  

Instead of having a single point of failure, now you have three.  

What if one of the servers goes down?  Now replication is broken everywhere.

How do you replace it easily and align the binary log positions?  You can't without doing some very complicated and error prone binary log analysis.

So, now you have to restore all three servers from a backup.  Wait, was that backup taken at the same moment in time?  Nope, because there's no way to make a transactional backup across multiple servers.  So, you're left with broken replication and attempting to fix it with pt-table-checksum and pt-table-sync...or rebuilding the entire cluster from scratch.

Just because you can do something, doesn't mean you should.  Don't do this, really.  I'm not kidding.