Should I ditch this relationship?

I have a friend that I've known for a few years and we enjoy doing things together. Recently she offered to let another friend of hers move into her house with her. Now whenever my friend calls me to do something, the other friend is included. I don't dislike the other friend; just not interested in spending time with her. What was once a comfortable duo relationship has become a trio and I am not interested in a trio. What are my options? Should I give up the relationship, or should I just be honest with her and tell her that I do not want to spend time with the other person, or can I even say that as it would be rude? If everytime she asks me to join her and the other friend, I say "no"; won't that be a dishonest and indirect way of dealing with the issue? This is a difficult issue as I want to keep the friendship but it has changed. Am I being petty and/or selfish by wanting it to be exclusive?