"While we maintained the highest safety standards possible, accidents unfortunately happen and it is impossible to guarantee they won't in the future. Accordingly, we have reached this difficult decision," HBO said in a statement.
On Tuesday, at the Santa Anita Park, where much of the show was shot, a horse was being led to a stable when it reared and fell back, suffering a head injury. That horse was later put down.
"We are relieved that no more horses will die for this television show," said Matt Bruce, spokesperson for People for the Ethical Treatment of Animals (PETA).
PETA first spoke out against "Luck" during the filming of the first season after two horses were hurt during race scenes and then euthanized.
"If HBO wants to do a show about horse racing, they should be using fit, trained horses that are healthy, and the person monitoring them should be someone that has their best interest in heart," said Bruce.
"Horses are very fragile. They do get injured and every now and then one does have to be euthanized, but it's rare," said horse trainer Mark Farndale.
Farndale teaches actors to ride horses for television and movies. He says in a production environment, the horses have to come first.
"Their DNA says, 'run.' That's how they've survived all this time, so you have to be very careful of that, that you don't trigger that instinct because they'll just take off. They panic and go," said Farndale.
The American Humane Association, which oversees animal welfare on Hollywood productions said in light of the three horse deaths on "Luck," HBO arguably made the best decision it could, "and it will now follow up with HBO to find out the disposition of the horses in the Luck barn and ensure that they are retired properly."
HBO will air the final two episodes of season 1, which is already in progress.