Edit Message Template Permission
I didn't mean to answer my own question but I kept poking around - I'm also not sure that this is the whole picture:

I found that the main "edit message templates" permission shows the "Message Templates" option in the Mailings menu. But with either of the other two permissions turned on, you can still access the page at yoursite.org/civicrm/admin/messageTemplates.

I also found that the ability to Enable/Disable a template is tied to the new permissions. The option is displayed but it throws an error if you try without the additional permission(s)


  API permission check failed for MessageTemplate/create call; insufficient permission: require edit message templates and edit user-driven message templates and edit system workflow message templates


Meaning you'll want to add these additional permission(s) to anyone who already had the previous "edit message templates" permission - so that they are still able to enable / disable templates.

If anyone has additional info please feel free to add!