What did Paul plead with the Lord three times to take away?

Answer: A thorn in his flesh

Paul was given 'a thorn in my flesh, a messenger of Satan' to keep him from conceit; three times he pleaded for it to be taken away.

“or because of these surpassingly great revelations. So to keep me from becoming conceited, I was given a thorn in my flesh, a messenger of Satan, to torment me. Three times I pleaded with the Lord to take it away from me.”— 2 Corinthians 12:7-8 (BSB)