Best way to define true, false, unset state

Boolean a = true;
Boolean b = false;
Boolean c = null;

I would use that. It’s the most straight-forward.

Another way is to use an enumeration. Maybe that’s even better and faster, since no boxing is required:

public enum ThreeState {
    TRUE,
    FALSE,
    TRALSE
};

There is the advantage of the first that users of your class doesn’t need to care about your three-state boolean. They can still pass true and false. If you don’t like the null, since it’s telling rather little about its meaning here, you can still make a public static final Boolean tralse = null; in your class.