Brazil were no match for the world's number one side, who also held Japan to a 1-1 draw in the tri-nation competition at the weekend, in a game held four hours early because of the dangerous storm coming in from the Pacific.
Carli Lloyd, Shannon Boxx and Amy Rodriguez all got on the scoresheet in an easy win for the U.S. who are desperate to retain their title at the London Games that start in July after being beaten by Japan in the 2011 World Cup final in Frankfurt.
Lloyd smashed in an 18th-minute opener from close range before Boxx headed home Lauren Cheney's free kick less than five minutes later.
Rodriguez completed the scoring in the 83rd minute after another Cheney free kick triggered panic in Brazil's defence.
U.S. coach Pia Sundhage told reporters: "We looked good at set pieces and that's something we talked about so I'm happy about that.
"Playing teams like Japan and Brazil is always good preparation for the Olympics."
Japan are ranked third in the world, behind the U.S. and Germany, while Brazil are ranked fourth.
Only around 500 people watched the game due to the storm warnings.
Brazil were missing five-times FIFA Women's World Player of the Year Marta, on club duty in Sweden.
"It wasn't our best squad so we knew it would be a tough match," Brazil coach Jorge Barcellos said. "We wanted to give some younger players a chance before the Olympics."
Japan take on Brazil in the tournament's final game on Thursday.