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 occurence 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 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.
List of errors