I don't think I can limit myself to just three. There are a lot of traits that are important, and there are more than three which I think are critical.

I think spiritual maturity is an important trait. It helps you to know what is right and to guide you through your relationships as well as through times when you do not have a relationship. Choosing this as a trait is kind of cheating cus it encompasses other traits like those AJ mentioned (prayerfulness, faith, and humility...perhaps others), but I think it's an important one to have.

I find intelligence/wisdom to be very important. This helps us to make wise decisions and to better understand what is right or what is the best decision to make in a variety of situations.

Self control is also high on the list of important traits. It's nice to know what the right thing to do is, but without self control it can be difficult to actually do it. Self control in a relationship will help you not just to keep things appropriate when opportunities to do otherwise arise, but also it helps keep you from getting into needless arguments when boys get on your nerves (yes, even a YF can easily get on your nerves).

I think success is also important. If you aren't modeling the kind of life a boy would want to have when he grows up, why would any boy want you as a role model? If he somehow ended up with you as a role model despite this, could you really be an effective one if your own life is a failure? I think it's important for us to become people worthy of being emulated if we are to invite boys to look up to us.

Patience is of extreme important as well. Most BLs seem to go through periods where they are without access to boys and getting through those periods requires patience. More importantly, interacting with boys requires a lot of patience.

There are others I think are important, but I'll stop there cus the post is already getting longer than intended.