If the game was to be historically accurate, Venice really wouldn't be able to do much at all. By this point the Venetians were so poor and had so little respect international that the Ottomans could basically do as they pleased with their remaining colonies. Trade in the Mediterranean just barely kept them afloat, if heavily in debt, and their army...well...if I recall rightly, they didn't provide much (if any) resistance when the Austrians marched in.
Portugal, though, I think, is a bit more different from Spain than they're given credit for. My general impression is that the Portuguese went for the 'small, professional' model army, against Spain's 'just put as many people in uniform as you can' approach. This was at home, of course. I'm not sure about the Brazilian army or other colonial forces.
Later, against Napoleon, the British helped train their army as well, giving them a British-modeled force.
Russia has two of the three :P
Best artillery corps in the world, and cavalry generally described as reliable, if not as flashy as other nations.
But don't mind me ;)
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