I would add NAD and Anthem to that list. I know Anthem has 3 ch amps.
Marantz was originally American, formed by Saul B. Marantz, but is now owned by Marantz Japan which is part of D&M Holdings (Denon and Marantz, who also own McIntosh and Boston Acoustics). As an owner of a Marantz receiver, I can say it is the best out of the 4 receivers (1 sony, 2 denon) I have owned over my HT years, but I do not think they have a 3 ch amp.