I think that it is very important early on in a relationship like this to set down some boundaries.
This includes things like allocation of time, and what it takes to change that on a temporary basis. Visiting arrangements - is it ok for her to come over to your place when you are not around? Is it ok for the two of them to have sex in your house, cook a meal, or other activities.
I assume that you have had the "safer sex"/testing discussion - you can search this forum for the concept of "fluid bonding" if you're not already familiar with it.
It is very easy with NRE for someone new to poly to go ga-ga and neglect their existing relationships. The important thing is for you to voice your needs when it comes to things like going out, or any other quality time. You almost need to help him keep his own feet on the ground.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb