Two of the fatal injuries occurred at Santa Anita, and one at Los Alamitos, the California Horse Racing Board reported.
"We would like this outlawed," said activist Heather Wilson. "And the way you would do that is by outlawing the gambling. Gambling is the lifeblood of this digusting, outdated bloodsport."
Twentyseventrouts, a 6-year-old bay mare with eight career starts and one first-place finish, was leading Sunday's third race at Los Alamitos Race Course when she suffered a catastrophic injury down the stretch and fell face-first to the turf.
She is the eighth horse to die from a racing or training injury at the Cypress track this calendar year.
Classic Coyne, a 4-year-old dark brown filly with 11 career starts and one first-place finish, died Sunday after suffering an injury while training at Santa Anita.
Animae, a 2-year-old bay filly who never raced, died on Saturday from a training injury suffered at Santa Anita.
Santa Anita has now had 11 racehorses die from racing or training injuries in 2023.
The latest fatalities come two days after the CHRB said increased safety measures led to a decline in the number of horses who died from racing and training injuries at board-sanctioned tracks during the 2022-23 fiscal year.
The board said 26 horses died as a result of musculoskeletal injuries in the fiscal year ending June 30, compared with 39 the previous year.
City News Service contributed to this report.