The site did use Tinymce but I understand this will not work now.
You can install additional editors (https://docs.grabaperch.com/addons/plugins/installing/). TinyMCE is listed in the add-ons archive. Not sure whether it works on Perch v3 though.
I have tried the others and none of them let the user update tables that I can see.
Perch v3 ships with Redactor. You can install the Redactor table plugin.