The trio all have strong racing pedigree in the United States, having each won the Indianapolis 500 on top of their Formula 1 palmarès.
Andretti, regarded as one of the most versatile drivers in the sport's history, also won the world drivers' championship in 1978 and the Daytona 500 in 1967, the only man to achieve the triple.
He is also one of only two Americans to win the world championship, the other being Phil Hill.
“It's an absolute honour for me to be one of the Ambassadors of the inaugural Formula 1 Crypto.com Miami Grand Prix,” he said.
“Although I achieved victories at Daytona, Indianapolis and in IndyCar, I was very proud of the success I achieved in Formula 1 — winning the world title with Lotus and scoring a podium with Ferrari at the Italian Grand Prix.
“I still love the sport of F1 and can't wait for the racing to begin on this fabulous circuit built in the campus next to the Hard Rock Stadium.”
Fittipaldi headed Stateside after his F1 career ended, having won the 1972 and 1974 world titles for Lotus and McLaren respectively.
He twice won the Indianapolis 500, first in 1989 and again in 1993, and boasts a street named after him in Miami.
“As somebody who has spent decades living and working in Miami, it's a huge privilege to be associated with this brand new F1 race in a place I call home,” said Fittipaldi.
“For many years I've thought how wonderful it would be for this great city to host a Formula 1 Grand Prix and now, thanks to all the efforts of the team at South Florida Motorsports, we have this amazing 19-turn circuit that raises the standards for all future Formula 1 events.”
Montoya's comparatively fleeting F1 career brought grand prix wins for Williams and McLaren, though the world championship eluded the Columbian.
He joined the European circus off the back of a successful stint in US open-wheelers, winning the Indianapolis 500 on debut in 2000, and again in 2015.
“Having lived in this part of the world for a long time, I'm very excited that we will be able to showcase all that Miami and South Florida has to offer,” said Montoya.
“This is a great place for music, food and sports and everyone who comes to this race will understand why it's such a special city.”
The Miami Grand Prix runs from May 6-8.