I have a MacBook Air 2017 with High Sierra. I tried numerous times to install Boot Runner 3 without success. I made a search of the files with EasyFind and discover that the files are put in the trash.
When Boot Runner installs over an existing install, it move any existing files to the trash. Try looking in /Library/PrivilegedHelperTools/ and see if you see Boot Runner in there.
There is no /Library/PrivilegedHelperTools.
Try opening Terminal and running this:
and send back the output.