HBO 'Luck' Horse Trainer -- 'Sometimes S**t Happens'

The man who trained the third horse that died on the set of HBO's "Luck" is telling friends ... NO ONE is to blame for the fatality ... insisting the animals received exceptional care, but sometimes "s**t happens."

Sources close to Matthew Chew -- who trained and owned horse #3 -- tell us the people who looked after the horses are some of the best in the business ... who thoroughly examined the animals at least twice a day during production.

Chew has told friends ... the horse was simply playing around when it "misstepped" and fell down Tuesday morning ... suffering a head injury that required it to be euthanized on the set.