Hello,

THe reason is 95% due to an extension conflicting with Joomla 3 jQuery library (that could mean an extension not coded for Joomla 3, but running on it, as for example, a plugin loading its own jQuery Library).

Try first to disable the editor buttons (if you have any additionnal buttons for your editor) and try with another editor (tinymce or JCE, which are working editors on Joomla 3).
If no change, so it's maybe another plugin (system...).
By disabling one by one not-joomla-core plugin, you can find which one is returning an issue.
This could be a module too, if you have one not-joomla-core in admin side.

If you know how to display the code source generated (right click on your browser page) you can check scripts files loaded in the head of the page, and check if one script file as jquery in its name.

Hope this helps ?

Best Regards,
Cyril