I don't see a particular reason why
Code:
SELECT champ_gid AS GID, champ_mid AS MID, champ_name AS name, COUNT(champ_mid) AS total FROM games_champs GROUP BY MID ORDER BY total DESC LIMIT 3
Would fail on MySQL 5.7, I wonder if the table was properly upgraded. Try running that exact command from phpmyadmin and see if it works or if an error message comes back. Check you can browse the table games_champs.