How do I get to third floor in Big Boo's Haunt?

Couple of ways to get to there, two ways to get up there. If you you already have the stairs, use those to get up. You can go to the left door on the back wall, and wall jump up to the grate, or you can wall jump on the left wall in the first room, about halfway in-between the door and the balcony (harder than wall jumping onto the grate, but possible). Go into the far right room on the second floor, and either use the power flower box to float up, or do what I do, get on top of it, face away from the door, be as close to the edge as possible, and back flip while holding the up button. Then go through the door!

There is 2 ways that you can get there.

The first way is to be Mario. Enter the mansion through the front door and go up the grey stairs if you have them. If not, look on the back wall, and enter the left door. Watch out for the eye, killing it if you wish to. Anyway, wall jump up to red wire grating. There will be a door somewhere up there, very easy to see. Go through it and you'll be on the 2nd floor. On the second floor, go to the right, and enter the farthest door. In this room, right in front of you (according to the view of the camera) there is a low platform type thing with a ? block on it. If Mario, hit the block, grabing the Power Flower. Float up, 'til you see a high platform with a door on it. When you see your shadow on the platform press R and ground pound. Enter the door. You're now on the 3rd floor! This is the way you have to go to get the stars "Big Boo's Balcony" and "Eye to Eye in the Secret Room."

The other way HAS to be done with Luigi. If you don't have the stairs, go to the same room; Back wall, left door (as Luigi.) Avoid the eye as usual. Stand somewhere in the room, anywhere you desire, and do a backflip. If you stood in one of the right places, you'll land ontop of the wire. Once on top of the red wire, enter the door again, where you'll come out on the 2nd floor. Go to the far right door again, and enter. Stand on the platform with the red ? block, face the door you just entered from, and backflip.

Congrats! You now know how to access the 3rd floor! P.S. If there is one section in this answer that you didn't understand, I highly reccomend reading a FAQ in the General section by Deathborn 668.