Why do I have 3 Macintosh HDs appearing in Disk Utility

Macintosh HD contains macOS

Macintosh HD - Data contains all User Account data. If there are two of them it means you created one additional container. It's up to you to find out if you want to keep it or not.

Perhaps the following will help explain: Add, delete, or erase APFS volumes in Disk Utility on Mac.

I would not dare to presume that I "know what I am talking about" according to your expectations; however, why don't you do a search here re. duplicate Macintosh HD - Data volumes. The second Data volume is quite often almost empty and it is also quite often caused by a reinstall. There are posts on how to eliminate it. Or you can simply use recovery to erase your drive and reinstall.

In Finder, there should only be one Macintosh HD showing. However, in Disk Utility, there should be both Macintosh HD and Macintosh HD - Data. That is new in Catalina and is expected behavior. The first contains the OS/system and is read only; the second contains apps, your files, etc and is read/write.

And, as an FYI: we are all other users here volunteering our time to help others; you want professional support, you need to contact Apple.

https://support.apple.com/en-us/HT208496