I guarantee you these three words are easy to differentiate; in fact, I can assure you of that.
ASSURE means to give others information that will erase any doubts they might have had:
“UCLA assured me the math section of the Graduate Record Exam would not count on my admission to the English Department Master’s program.” (This is true. I called twice to make certain. That’s how good I am in math.)
ASSURE also means to make something sure to happen:
“Samantha assured her passing the DMV written test by assiduously studying the boring rule book and by passing all the practice tests.”
Use INSURE when you are dealing with money:
“You insure a package at the post office, you insure your car against theft and liability, you insure your home against fire, theft and earthquakes.”
ENSURE means a guarantee that something will be done:
“Harry ensured the principal that he would stop feeding his homework to the dog.”
You see that ASSURE and ENSURE can often be used interchangeably, but keep INSURE for when money is involved.