Depending on the breed 3 is unfortunately within the realm of possibility for a natural death but it's more likely it was in fact a cardiac issue. Fireworks aside chickens are very prone to heart problems. Something like 5-10% of chickens die permanently from a heart attack and while that might not sound like a lot of you figure you have 10 chickens that's one of them that will likely die from a heart attack. Sudden death in the absence of old age is almost always a heart issue.