Triple Decker Buses?

Yes. You just need to rebuild every bridge in the world, along with half the traffic lights, and then make the wheel track twice as wide. Otherwise probably not. Doubledeckers are already rather top heavy, and have enough trouble fitting under bridges as it is. Adding three or four tons two or three meters higher really isn't going to help things.
 
Imagine you're on the top deck. You've dosed off, and come to just as the bus comes to your stop. Now you've got to run down two flights of stairs to get out.

I once didn't make it out of a crowded two decker.
 
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