I'd hazard a guess that there's no significance to it being 3 bars, regardless of orientation. You simply need to be able to look down at the panel and see something which indicates which button is selected..TBH it could just as easily be dots or stars, whatever......
FWIW on some Boeings you simply get a backlit illuminated "MIC" where appropriate.
Last edited by wiggy; 2nd July 2017 at 13:22.