It is necessary to install Microsoft DirectInput Mapper[community.pcgamingwiki.com].
After that you will be able to configure input settings and reassign the buttons.
Default controls from PS2.

Need to download widescreen fix[thirteenag.github.io] , after that you need to unpack the archive to to "Onimusha 3" folder.
It is also possible to display videos and interface in 4: 3 format, for this you need open Onimusha3.WidescreenFix.ini in scripts directory, change "FMVFullscreenMode = 0" and save.
For fullscreen you need use command "-fullscreen" in steam game properties.

I also reduced the volume in BGM, these files can be downloaded here[drive.google.com] .