To insure something is to buy insurance. To assure is to express confidence that the outcome is as desired. To ensure is to do something to make certain tyhe outcome is as desired. Mary bought insurance in case her house burned down. The mother tried to assure her child that the lost cat would come home eventually. He filled up the gas tank to ensure that he wouldn't run out of gasoline.

insure = to cover and protect and underwrite assure = guarantee, comfort, promise ensure = to assure and insure