It sounds to me as though the three phases seem structured to introduce and explain different aspects of the game. I would imagine the first phase will primarily be about learning how to manage and build up your colonies on the campaign map and handle diplomacy with the natives, so I imagine large scale fighting is unlikely. The second phase sounds like it would be the natural place to introduce land warfare, against the Native Americans and the French. Then I imagine the third phase, the war against the British, will be the "apply what you've learned and bring it all together" phase.