It looks like it installed just fine. Are you asking where the installed commands are? You can query that using pacman if needed:
pacman -Ql mingw-w64-x86_64-mrtrix3
That will list all the files included in the package and where they were stored on your system. But these should all be in your PATH already, you should be able to start using them straight away. E.g. if you type mrview, hopefully the viewer will open up.
If you’re struggling with where to start, I suggest you take a look at the beginner tutorial to get a feel for how things work, and have a go at the BATMAN tutorial when you want to take it further.