You should neither talk to him nor wait for him to come back. You have been in the relationship for a lengthy period of time.
One thing that is evident is that for the past 3 years, he has been thinking a lot of the girl. This means his feelings for you were not the same before he met the girl. It simply means the feelings he has for the other girl has overshadowed the feelings he has for you.
You should accept the two of you to break up. You will feel hurt but it's the right decision to undertake. You should move on with your life.
You will get a man who loves you to the core. A man who has feelings for you only. That is what relationship is all about.
He has to 'fight' the feelings he has for the girl. When I love you, I have to ensure any feelings I have for another girl doesn't come close to the feelings he has for you.
Since he wants to clear his head by dating the girl, you should your head by moving on with your life. Never look back, never wait for him and don't accept to reunite.
If you wait for him, you will be hurting yourself. You have a life before you. Life is short, don't let it pass you by.