To me the term Mt Rushmore is about what established the brand not what is current establishment.
Sony: Crash, Lara Croft, Solid Snake and Cloud
Microsoft: Masterchief, Marcus Fenix, (Honestly not really anything outside of this comes to mind)
Nintendo: Mario, Link, Donkey Kong and Mega-man
Good choices, I actually agree with almost all of this. I'd probably replace Mega-man with Samus if I were to change anything, but I can definitely see why you chose Mega-man too.