These three overlap in meaning just enough to confuse a lot of people. Few of us have a need for “incidence,” which most often refers to degree or extent of the occurrence of something: “The incidence of measles in Whitman County has dropped markedly since the vaccine has been provided free.”
“Incidents,” which is pronounced identically, is merely the plural of “incident,” meaning “occurrences”: “Police reported damage to three different outhouses in separate incidents last Halloween”.
Instances (not “incidences”) are examples: “Semicolons are not required in the first three instances given in your query.” Incidents can be used as instances only if someone is using them as examples.
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