There are two possibilities for the final round interview. One is that the hiring manager has picked the candidate and the senior guys use the final round to confirm the selection as quality control. The other possibility is that the hiring manager presents several candidates for the senior guys to make the final selection. It will be userful for you to know which one is your situation.
For the first situation, the senior guys generally want to know your interpersonal skills, relationship skills and professional skills at the high level. You want to be more relaxed, ask questions, pay attention to their vision and interests. Let them do most of the talking.
For the second situation, you need to be more aggressive in presenting your "brand", somthing that can set you apart from the others. You may want to think about your strength and use a few key words to capture how you are different form others. You still want to be relaxed and delightful, but you also need to consistently use the key words to present your brand.
Good luck.
