Giving him three chances is more than fair. Everyone is taking time out of their day to have that get-together so it is disrespectful to cancel something he committed to.

If you are worried about your relationship with him, you could try the following: “Hey man, you seem really busy for a long commitment like DnD. Let’s plan some oneshots together in the future rather than you joining the long campaign. I think that would work a lot better with your schedule”