Answer:
The three longest rivers in United States are the Missouri River, the Mississippi River, and the Yukon River.
Explanation:
Commas in this case are used to itemize a series. In the example sentence, the subject is on the 3 longest rivers, and the items are "Missouri River", "Mississippi River", and "Yukon River". As such you will use a comma in between each of them.
Learn more about commas, here:
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