Assure means to tell another person something to remove doubt or anxiety.
After assure, we always have a person: assure you, assure him/her, etc.
Ensure is something you do to guarantee a specific result.
Insure (spelled with an “I”) is when you get a financial plan to pay for any damage or loss to a person or thing. This is related to the word insurance, such as health insurance, car insurance, etc.