Assure means to tell another person something to remove doubt or anxiety.
I was afraid we'd miss the flight, but my husband assured me we'd get to the airport in time.
I assure you that the water here is perfectly safe to drink.
After assure, we always have a person: assure you, assure him/her, etc.
Ensure is something you do to guarantee a specific result.
We ordered ten pizzas to ensure that there would be enough food for everybody.
Please send the document by express mail to ensure that it arrives on time.
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.
Our house is insured against fires, floods, and theft.