Im going to asume you have the easy one to the left of the main path, blow up de builer and step on the button.
Then for the last two: in the area where you're completely surrounded by doors (follow the path to the left that leads away from the last area with the teleporting guy) you have to beat all the monsters which will open a door, beat all the monsters there and get star number 2.
Then through the door to the top area, there is a higher button there, use your hammer on that one (if you dont have the hammer, come back later if you do, or you can try the churro gun and shoot at the cage with the star if you have that, that will occasionally work too) and a door to the left will open. And thats three stars.
Then for the last two: in the area where you're completely surrounded by doors (follow the path to the left that leads away from the last area with the teleporting guy) you have to beat all the monsters which will open a door, beat all the monsters there and get star number 2.
Then through the door to the top area, there is a higher button there, use your hammer on that one (if you dont have the hammer, come back later if you do, or you can try the churro gun and shoot at the cage with the star if you have that, that will occasionally work too) and a door to the left will open. And thats three stars.