Tifa, what do you mean when you say that you want to "do the 3 hour drive around the island"?
Poster "capebreton" has given good driving time estimates. I have seen his/her posts here b/f & "capebreton" seems very knowledgeable about Cape Breton Island.
You shd actually allow for a "fog day" when going around The Cabot Trail....if the day you plan to "go around the Trail" is foggy, there's no point going b/c you won't be able to see anything....not even your hand in front of your face!
Mid-Sept is a perfect time to visit Cape Breton Island but it is too early for fall foliage. You can generally expect beautiful weather...lots of sunshine, blue skies, breezy( but you can get fog & rain too!)