Although these three often show up at the same party, giving hugs, they're not the same, thank you very much. To assure is to tell someone everything's OK, to ensure is to make certain, and to insure is to protect financially. Have it straight now? Are you sure?

To assure is to remove doubt, or confidently tell someone about something. It's generally followed by an object, so you assure someone. You can also tell someone to rest assured when you know everything's under control:

"Rest assured that Brazil will have a great World Cup in 2014," Rebelo said. (Washington Post)

I assure you I meant no harm. (V for Vendetta)

He considered retirement before doctors assured him he could still fight. (Newsweek)

Ensure, on the other hand, means to make certain a thing will (or won't) happen:

Aides said the leaders conferred by telephone to ensure that their speeches, while different in tone, would not be incompatible. (Reuters)

Roast Brussels sprout mixture on sheet pan for 12 to 15 minutes, shaking pan about halfway through to ensure even cooking. (New York Times)

The "feed kids first" policy is designed to ensure that parents can eat in peace. (New York Times)

Finally, use insure when you need insurance. To insure is to arrange for financial compensation against the loss of something or against someone getting hurt or dying. You might insure your health, your Cadillac, your beachside condo, or your stocks and bonds:

Very well then, listen: You know our house was insured for a good deal of money — fifteen thousand dollars. (Gabrielle Emile)

Euro-area finance chiefs meeting tonight also will discuss using the European Financial Stability Facility to insure bonds of troubled governments. (Business Week)

Remarkably, younger and healthier individuals in other industrialized countries have long accepted the mandate to be insured in return for community-rated premiums and guaranteed issue. (New York Times)

Sometimes people say ensure or insure when they really mean assure, to remove doubt. Bryan Garner points out in Garner's Modern American Usage that the usage has become "ubiquitous." Harrumph! We assure you that there is a difference, and by reading this you've ensured that you know your homonyms, but alas we can't insure you. It's just not that kind of party.