USA, Mexico, and Canada will be team #1 in already known groups (potentially Groups A, B, C for example). It will already be known, before the tournament is even drawn, what venues that the hosts will all play their group games at, and yes, US will play in the US, Mexico in Mexico, and Canada in Canada. Mexico and Canada's groups will likely also feed into second round games also played in those countries, but eventually if Mexico and Canada advance far enough, they will play games in the US. The US will likely have all possible permutations of games be in the US.