After the events of Zero Hour, the USA retreated to North America to defend the homeland. If they stay that way then I’m not sure how much of a major player they would be.
The tank in the trailer has “EU” painted on it, which confirms the EU as a potential faction. The Quad cannon trucks have “GLA” painted on them which confirms GLA as a faction.
The question is, what would the third faction be? With a Western and a Terrorist faction, the only remaining faction would be an Eastern faction. I would guess that it would logically be China because they were the canonical victors of Zero Hour.