However who in in the Americas could put up a good resistance?
The Aztecs , when defeated did not rise up as the Maya and Inca did, however they actually blended into the new society of Criolle, Mestizo, and Mexican... so I would rule them out... they stopped being a crucial military power, instead they became mercenaries and native-conquistadores... though most became slaves or died...
however I think the 3rd playable nation would be Maya or Inca
why Maya or Inca?
because both Maya and Inca have both risen up successfully in reconquest... though ultimately could not keep up a martial resistance due to disease (if more than 90% of your people died within a year you'd understand)
Why Inca?
The peoples in the Incan Empire were united, well cared for, and had a true empire. They had roads, complex forts, complex warfare and politics and also they kept care records and cared for their people and also mobilized labor in order that the highlands could, with proper terracing and irrigation, become arable.... this was very important...
the Incan Empire, although relatively young, was extremely powerful. Infact Manco Capac I believe used extremely brilliant tactics such as releasing dams, making forts, and capturing fire-arms, to drive out the Spaniards though ultimately military means failed.
Why Maya?
The Mayans had, by the time of the conquistadores, no large cities like Tikal, Uxmal, etc. Instead they lived in well hidden fort-cities and many villages scattered throughout the Yucatan, the Peten forests and the highlands...
they have a very varied overall 'culture' since highlanders were very influenced by Aztecs and in Yucatan by Toltec descendents, the Itzas... also they used traps and guerilla tactics to fight the Spaniards., They, like the Inca, nearly were successful in driving out the invaders. This Caste War was amazing and they stopped as they needed to return to their villages to plant the corn...
so having said what I think, what do you guys think?