I have 3 sites that I want to run on a JMS instead, how can I move these sites into JMS?

Move existing standalone websites into slave sites is possible with manual processing.

The idea is to check that the master website has installed all the extensions used by all the standalone websites.
When you are sure that the PHP code of all the extension are present in the master, you can create slave site that will be used to receive the standalone DB connection.
Once the slave site are defined, instead of creating a fresh slave site, you can just re-use your existing standalone website "configuration.php" file that you put in the "deployment folder" of the slave site.
Perhaps you will have to ajust the "configuration.php" file to setup the new location directory.

When done, you should be able to connect on your slave site.
The only thing that you probably still have to copy manually is all the images and files that are potentially used by your standalone website.