Problem remotely connecting to MySQL database
There could be multiple reasons for this but to start troubleshooting as it looks like the issue with permissions/grants.

1.Please check and assign proper grants on server 1 :

GRANT ALL PRIVILEGES ON db.* TO 'username'@'server3_ip' IDENTIFIED BY 'Password';

flush privileges;


All is not compulsory in above query.

2.Double Check Bind parameter in in My.cnf .

bind-address 0.0.0.0


Hope this helps else we can debug further.