Copied Scores
Every time I make a change to a score and save it, Musescore creates a new file and renames the old file to .[score name].mscz,. Musescore will not open the renamed file and I get tired of always having to delete useless files. 
Here's what it looks like.
This also happened in Musescore 2.2 sometimes but not always. I have the latest version of Musescore 3 (I updated today) and I'm running it on Windows 10.
Comments
Those are backup files of the last saved file. The "new" files are the latest ones you saved.
If you wish to open the backup file, remove the period before the name and the comma. You will have to rename it to avoid conflict.
In reply to Those are backup files of… by xavierjazz
So there isn't a way to stop saving those backup files?
In reply to So there isn't a way to stop… by Aaron Stalter
Not.
In reply to So there isn't a way to stop… by Aaron Stalter
Don't bother. They'll just keep being replaced (unless you change the name). One day they may be of use to you.
In reply to Don't bother. They'll just… by xavierjazz
I'd wish I could save them to a different folder than where I save my "real" scores but it doesn't look like you can.
In reply to I'd wish I could save them… by Aaron Stalter
Also, saving them in the same folder as the existing score makes finding them easier to find and eliminates any duplicate naming issues, like I have two scores with the same name in different folders at the moment.
In reply to So there isn't a way to stop… by Aaron Stalter
If something tragic happens to the last file you saved intentionally, you will be glad the backup is there so you have less to recreate. The nice things is the backup is replaced rather than having one for each session.
Actually, it did happen in 2.2 always, and in every version before that. The files are normally hidden by default; you must have enabled display of hidden files in Explorer. Turn that option back off, at least for your Score folder, and you won't be bothered by them. They do serve a valuable purpiose, you are better off not deleting them.
Background for those not familiar with this stuff. Many applications (Microsoft Word, Powerpoint, Emacs, vi, Musescore, etc., etc.) have an "autosave" and an "autorecover" file feature. Autosave is a copy of your file before you started your current editing session. If your app crashes, you'll have the autosave file, but lose your work done in the current session. Autorecover is the app's best attempt at saving a "temporary" file that it creates and updates within the session. Details vary.
Naming conventions, and locations for the autorecover files, abound...
I agree with others that it's a valuable feature, so I recommend tolerating it.
Autosave:
I don't know why Musescore used that loony comma at the end. The foo.mscz ---> foo.mscz~ naming convention would have been less scary than foo.mscz ---> .foo.mscz, Both the "." and the "," are hard to see. I think that a configurable location for foo.mscz~ would be good. Easy enough to deal with the collision problem using a time string in the name.
I understand that the "." at the start of foo.mscz ---> .foo.mscz, was to enable Windows people to hide those funny files, but the consequence is that half the users can never see or find the damned things :(
Autorecover:
I don't know why knowledgeable people here don't direct score losers to these files. I don't know enough to risk telling people the wrong thing. On my Windows box, they are in a "session" folder, with names like:
C:\Users\myname\AppData\Local\MuseScore\MuseScore2\scEwo652.mscz
In reply to Many applications (Microsoft… by MikeN
The problem with autosave files, that start SC is that they are deleted when MuseScore closes normally. If you find one after a normal shut down, it will be old and probably not what people are looking for.
As for allowing the user the option of where to put .foo.mscz, files, if you have 2 files with the same name in different folders like I do, you would have files overwriting the wrong files. This is not possible if the backups are in the same folder as the main file. Backup files are invisible by default. If you can see them, you have gone through effort to see them and should either turn off the invisible files or live with the comfort of knowing they are there if you need them. I occasionally make a stupid mistake, like deleting or overwriting the wrong file. I use the backup to minimize the work necessary to restore my file.
In reply to The problem with autosave… by mike320
All right, I understand know what they are and why I'm seeing them. I was tracing some programs to their roots in File Explorer and needed to get into the AppData folder so I turned on hidden files and never bothered to turn it off since I need in there (and other invisible folders) quite frequently. Someone mentioned that I can set just my Scores folder to not show invisible files while File Explorer still shows them everywhere else. I wasn't aware of that, but I'll look into it. Thanks for all the help!!