Help! 3 of my horses have recently ped away for unkown reasons. What could it be?

My first horse, Joey, ped almost three months ago. He appeared to be in perfect health. He was up to date on vaccinations, teeth floated, worms, everything. He was very old though, almost 30, so we chalked it up to old age. Less than a week later my pony coliced. We raced her to the vet and luckily she was able to p a stone and returned to normal. Then a couple of weeks later she died unexpectedly for no apparent reason! We thought at the time it was maybe trauma from the colic or another bout of it? That left me with one pony who died this morning with no symptoms. She was 10 years old and was eating regularly and was in good health. Im so scared and frustrated. We never changed their food, pasture, or routine. What could have caused this to happen? Why would seemingly healthy horses die within months of each other?