The 29-year-old rider from Scotland and his 13 year old horse won the Grand Prix in Switzerland in December, the World Equestrian Festival in Germany in May, and completed their historical accomplishment at the CP International in Calgary.
Remarkably, the duo was also the first to win consecutive events, let alone three in a row.
For his efforts, Brash received a prize of $2 million for the win at Spruce Meadows.
"If it could be done in Calgary, if I could win, it would be an absolute dream come true," Brash said in an interview before making the voyage to North America.
This accomplishment is regarded as a high-water mark in the 250-year history of the sport.
Foster named top Canadian at Nation's Cup
Tiffany Foster was named the Mercedes-Benz Leading Canadian Rider on Sunday to close out the 2015 Spruce Meadows Masters tournament after gaining the most points in the 1.4-metre division.
It all came down to the final day of competition for the North Vancouver, B.C., native.
Going into the final class — the $25,000 JLT Cup — Foster was tied for the top Canadian with Lisa Carlsen of Spruce Grove, Alta., while Calgary's Ben Asselin was one point behind.
All three riders qualified for the jump-off, but in the end Foster prevailed, placing third in the field and finishing ahead of Asselin in fourth and Carlsen in fifth. Samuel Parot of Chile won the competition and Ireland's James Chawke placed second.
"There were a lot of good riders in that division, and it came down to a nail-biter in the jump-off," said 31-year-old Foster, who was part of Canada's recent team gold-medal effort at the Pan American Games.
Foster added: "It's a really big deal to win this award at Spruce Meadows. Every time we come here, we want to be good, no matter which division we are jumping in."
Foster was a member of Canada's third-place team in Saturday's $300,000 BMO Nations' Cup, riding alongside Yann Candele, Eric Lamaze and ten-time Olympian Ian Millar.