I hope you've been looking for other work, focusing on just one company isn't a good idea.
Also, I don't know why you would email the CEO, that's very silly.
People outside of HR interview you to see how you would work in their team. Clealy the 3rd interviewer didn't think you'd be a good fit with the group. I wouldn't keep trying harrass them for answers as they won't disclose why they didn't hire you nor will the decision be overturned.
Take the interviewer's advice and reapply in 6 months.
Best of luck.