The three largest mountain ranges in the United States are the Pacific Mountain System, the Appalachian Mountains, and the __________ Mountains.

The three major mountain regions of the United States include the Pacific Mountain System, the Appalachian Mountains, and the Rocky Mountains. The Andes are in South America, the Cascade Range is part of the Pacific Mountain System, and the Canadian Shield is not a mountain range. Therefore, the best answer is the Rocky Mountains.

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