Well, the reason the Civs in Civ 3 were chosen to be in teh game, is because they're all the famous civilizations throughout history that made a large impact on teh world. I mean, the Celts are in the game because of they reached all over Western Europe in their peak, and we still celebrate one of their religous ceremonies (Halloween, which, despite what religious fanatics will tell you, is NOT demonic) and its been speculated that if they hadn't been fighitng each other so much, they could've conquered even the mighty Roman Empire (Another famous Civilization) of course, i gues they sorta did indirectly with teh Garmanic tribes over time. We all know about Egypt because of its incredible acts of masonry and fascinating religious beliefs, America and Russia are in teh game because they are currently big world powers.
The reason there is no "Canadian" civ, or "Australian" civ, or anything of that nature is because those countries/cultures neevr really had any great impact. I mean, Canada is a cool country (Hockey!!) and Australians are some of the tougher guys out there, but these countries neevr really had any major historical impact, and assuming Humans are still around in 500 years, and history hasn't been re-written by the politically correct squad, or anything of that nature, America, Russia, Japan, Germany, China, we will still be reading about all of these civilizations, while other more minor civilizations/countries will probably still remain just sidenotes in history.
Also, while we're here, does it seem strange to anyone else the Zulus are a main Civ in the game? I mean, SHaka and the Zulus are hardly ever mentioned in any history books, and while anyone who wants to fight off an oppressive rule is cool in my book, i think that a lot of the reason the Zulus were included was just for a politically correct representation of all races, i mean, we had Asians, Europeans, and American Indians, so i think they might've just decided to include an African Civ in the game.