Get Mario:
After you have 8 stars go in the door in the room that you can play rec room mini games and jump in the Mario picture. The level is easy! To beat Gomboss, (since you are yoshi.) run in front or behind him and put a goompa in your mouth and shoot it at him. Repet this 3 times.

Get Luigi:
Go to Big Boos Haunt level as Mario and go to where big boo's balcany would be(In the house up the stairs far door on the right use Mario to float with the flower up.) Jump in the Luigi picture.

Ok after a few tries you see why you hate this level. So go right,left,right,left and up to face the big boo. I found the way out of this level by following the boo's laugh. to beat the boo just punch him 3 times. when he is in the mirror just punch his shadow.

Get Wario:
Use Luigi and go to the mirror room(beat 2nd bowser go in the next door up and go right in the closest door.) go through the flower in the mirror and into the Wario picture. You should beat the level after a few trys. To beat the ice Bully just knock him in the water. punch him a few times to get a coin.