Ford and General Motors, certainly. But Fiat, although it has "a long history in the United States" (according to Wikipedia), has never been an American company.
The original 'Big Three' were Ford, General Motors and Chrysler – all based in Detroit. From 1998 to 2007 Chrysler was owned by Daimler–Benz, which renamed itself DaimlerChrysler. It then operated alone for seven years, before being acquired by Fiat and becoming a subsidiary of the newly formed Fiat Chrysler Automobiles (FCA) in 2014. In 2021 FCA merged with Peugeot to become Stellantis; Chrysler is still its American arm, so it could still reasonably be described as one of America's Big Three – and indeed it is, by Wikipedia.
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