I've never actually seen a utf8mb3 charset in the wild (it's the same as utf8 but it never calls itself utf8mb3). I would probably:
- do a database backup
- Run the api System.utf8conversion to convert to
utf8mb4. If the database is large, run it via command line e.g.cv api3 System.utf8conversion, otherwise you can also run it in the UI via api3 explorer with Job=System Action=utf8conversion
utf8mb4 is the default on new installs now and definitely recommended over utf8. And it usually fixes this "row too large" error but even if it doesn't you want to be using utf8mb4.