Transition multiple independent WP sites to one WP multisite on the server with minimal downtime

I currently have three independent WordPress blogs hosted on a single shared server.

The main domain (we’ll call domain-A is sitting in the public_html folder. domain-B and domain-C are separate WP installs residing as subfolders within public_html. Each of the installs is mapped to its own, unique URL.

Here’s the structure of the server:

/public_html     ..core WP files, etc     /domain-B         ...WP install core files     /domain-C         ...WP core... 

I am attempting to set up a WP multisite, incorporating all 3 blogs under domain-A as my main parent site.

My plan is to create the multisite in another subfolder in public_html and once the site is configured, I want to seamlessly – with minimal downtime – swap out the independent sites for the one multisite.

How would I do that?

Here’s what the new server config might look like:

/public_html     ..core WP files, etc     /domain-B         ...WP install core files     /domain-C         ...WP core...     /new_multisite 

Ideally, it would be great if I just had one wp multisite install in the root folder and could remove the other independent installs, e.g:

/public_html     ... wp multisite core files, etc.. 

I read some stuff about configuring my local hosts file on my system to be able to re-route the IP address to a domain URL (still a little fuzzy on some of the details of that) but I don’t see how that helps with a live site and/or a remote server.

Also, is it safe to create a "sandbox" site in a subfolder on the shared server? How can I keep this folder undiscovered or inaccessible? I am considering doing the setup on my local computer using WAMP as a dry run but I’ll still have to contend with uploading it to the live server, testing it, then making the switch from 3 independent blogs to one multisite.

Edit: I’m learning now that .htaccess can be used to control redirects to a subdomain, so theoretically, all requests to the original domain(s) can be redirected to sites/domains within the multisite. Obviously, the specifics of this starts to get a little hairy.

Thanks in advance,