Text Editor not working

Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the Twenty Eleven theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.

I will do those things but why is this happening for three different sites?

At the moment, I have no idea. They could be 3 different issues or 3 sites with the same issue. But you have to start the troubleshooting process off somewhere and the steps I listed above do that by focusing on the most common root causes.

None of those things worked.

Try adding define('CONCATENATE_SCRIPTS', false ); to the bottom of your wp-config.php file (just before the require_once line).