The three athletes were among 10 people who died as two helicopters filming a reality show crashed in a remote part of Argentina on Monday.
As France awoke to the news, the country's political leaders and best-known sports figures registered their shock and expressed their condolences on television and social media.
The French sports daily L'Equipe's website carried a picture of Muffat holding her gold medal in the 400-meter freestyle at the 2012 London Olympics alongside the words, "French Sport in Mourning."
French President Francois Hollande spoke of his "immense sadness," while the secretary of state for sport, Thierry Braillard, said, "French sport has lost three stars."
"Some had finished their careers and one was dreaming of gold in Rio (in 2016)," Braillard said on BFM television. "I'm profoundly saddened."
The helicopters apparently collided in mid-air in La Rioja province, about 730 miles (1,170 kilometres) northwest of Buenos Aires. Eight of those killed were French, the other two were Argentine. Authorities were at the scene of the crash late Monday trying to determine its cause.
"The world of sport and the Olympic family have lost three of their key members," IOC President Thomas Bach said, adding that the athletes "were all not only champions in their sport but also contributed greatly as role models."
The Olympic flag will be flown at half-staff for three days at International Olympic Committee headquarters.
Michel Platini, the president of European football's governing body, said "my thoughts are with them on this dark day for French sport, but also with the families of the other seven other victims of this tragic and terrible accident."
France rugby coach Philippe Saint-Andre also offered his support to the families of the 10 victims.
Muffat, who was 25, also won a silver medal in the 200 freestyle and a bronze in the 4x200-meter freestyle relay at the London Games. She retired last year.
"She had dedicated a lot of her life to swimming to become Olympic champion, and her objective since her retirement was to make a success of her (personal) life," her friend and agent Sophie Kamoun said. "She had a lot of projects that made her happy, and this show was one of them. I spoke to her on the phone two days ago and she told me she'd spent a fabulous week, one of the best of her life."
Kamoun recalled how Muffat was eager to take part in "Dropped," the survivalist television show she was filming in Argentina.
"She didn't hesitate for one second, things were going well there," Kamoun told BFM. "We knew she was happy there and that was the main thing."
Another French swimmer, two-time Olympic gold medallist Alain Bernard, was also a participant in the show and had been due to fly in another helicopter on Monday.
"The production company called me first, around midnight, to tell me the news and Alain called me a short time after. Obviously he was in tears, traumatized by what he'd seen," Kamoun said. "He told me he saw some flames and he knew it was dramatic."
Fabrice Pellerin, Muffat's former coach, spoke with pride and emotion about a recent encounter he had with Muffat.
"We spent a good time together, we ate at the restaurant, we shared the same table, we chatted," he said. "Right now I'm thinking about Camille. She was always the first to wish me 'Happy Birthday.' Always smiling. She was a fantastic person."
Vastine will never get the Olympic gold he craved — and one he felt was unjustly denied him.
The boxer won a bronze medal at the 2008 Games in Beijing after a controversial loss to Manuel Felix Diaz of the Dominican Republic in the semifinals. Vastine was ahead in the bout, but was docked points by the referee in the final round. He broke down in tears after the defeat.
There were tears again at the London Games four years later when Vastine drew on points with Taras Shelestyuk of Ukraine in the quarterfinals, but lost on the count-back rule.
"It feels like I've lost a member of my family. He was my friend, he was like my little brother," said Dominique Nato, the former technical director of the French Boxing Federation. "I had a lot of respect for him, no one will forget him."
Arthaud, who was 57, was a pioneer in sailing. In 1990, she became the first woman to win the famed Route du Rhum race — a trans-Atlantic single-handed yacht race between Brittany and the Caribbean island of Guadeloupe — on her boat Pierre 1er.
"She was a fighter," said French sailor Jean-Luc Van Den Heede, who was second in the Route du Rhum race in 1998. "At the time it was extraordinary because not many women were doing this. She opened the way for others."
French sailor Loick Peyron, winner of the Route du Rhum and the Jules Verne Trophy for the fastest circumnavigation of the world, bid Arthaud farewell on Twitter with a simple yet affectionate message: "We'll miss you Mimine."
Canoeist Tony Estanguet, a three-time Olympic gold medallist , said he felt "devastated" and "so sad for my friends," while former French sprinter Muriel Hurtis said, "I don't have words to express the pain I feel, the tears are flowing and won't stop."
Former France and Arsenal striker Sylvain Wiltord, ice skating champion Philippe Candeloro and veteran cyclist Jeannie Longo are among the other French athletes who took part in the reality show, but none were involved in the accident.
"I am sad for my friends, I'm trembling, I'm horrified, I don't have words. I can't say anything," Wiltord tweeted.
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