Quote from Mr. Piggy:
Neskamikaze: Technically, you only need to get the last star coin if you're going to the regular exit, since you will have already gotten the first two coins from the first time through to the secret exit, but I guess that's only if you're using my 100% route...
Oh, that makes sense. I wasn't taking routes into account, I was just going for that level.
What is your route? Does it go to Bowser as quick as possible and then go back through?