Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

Regarding with your issue, that was really hard for us to tell you why your site suddenly hitting 500 error without access your site to troubleshot this issue. It might be hitting one of the PHP fatal error then caused the 500 error on your site,

I checked all of my devices and found no viruses but the scan of my hosting server found three infected files. I have attached the screen shot of the files.

May i know your hosting server virus scanner did show you how long this virus infected these files? Because it might be other possibility infected your file in earlier from your computer before you upload installer into your Joomla backend.
If the computer already having these Trojan virus, it might be infected in installer file.

May i know your current site is it still having 500 error now? If yes, it would be best if you can provide us with your Joomla backend and FTP access so we can look into this issue directly.