Thank you for your question.
Sef Translate - Joomla translation extension works (translating) with HTML. It doesn't matter what the site you have - 3 Multilanguage-site or not. Sef Translate Joomla translation component allows to manually translate any part of the content of your site from Sef translate Joomla admin area. Sef Translate Joomla translation component will work on any Joomla website.
And what about your users? How they will navigate on website with Sef Translate and your Joomla Multilanguage-site. Note, You can check also free version with limited function and test it.