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Settings - Sound then Volumes - When changing volumes there should be a little icon saying which volume you are changing
i encountered the same problem, my previous phone can adjust all volumes.
however, there's a solution, if you can root your phone, install xposed, then install gravitybox tool. then you will be able to adjust all 3 volumes with the buttons.
but root your phone at your risk.
i also add temperature and download speed in status bar with xposed.