Homonym Hump Day - Ensure or Insure? And what about Assure?

Ah, my good old friend, embarassment. I know the definitions of these three, so why then, during an editing pass did I have to go out and look this one up? In this case I turned to Grammar Girl to set me straight. it was interesting to read that some may suggest all three words are interchangeable, but I was taught to think in terms of context when using them. How about  you?