Incidence is the act of something or an event occurring or happening. It usually refers to the frequency of something.
"High incidence of sports injuries among high school students has worried school authorities."
Incident is an event or occurrence that causes an interruption or a crisis.
"I am a victim of a rape incident."
Instances is the inflected form of the word, instance which means one of a series of recurring occasions, cases that are essentially the same.
"For instance, I wouldn't undergo a trial if I killed you on the basis of self defense."