I have a sis.-in-law who is great at this. She had a boy/girl shared room and did it brown, pink, and turkoise. She used brown as the "shared" color and then pink for the girl and turkoise for the boy. I have seen their room and it really works well. From what I've seen her do, I would say, 1) choose a neutral color for the "main" color of the room (whether it be for the walls or main color of bedspreads or whatever). In her case, this was brown; in your case, maybe the yellow. Then, choose two secondary colors that will go all right together, but will highlight the individual genders and "separate" them a little more. (Maybe purple for her and blue for the boys... there are a lot of combinations that would work, but some that would clash. A lot will depend on the shade of purple, etc.) For my sister-in-law's secondary color, she had their names on blocks on the walls, and had found material for blankets that was brown with pink polka dots and brown with turkoise polka dots. Obviously things like shelves, toss pillows, rugs, or smaller pieces of furniture could be done with the secondary colors. It was simple, but it seemed to work. I am not one for mixing a lot of different patterns, so my personal advice is to stay away from that and keep it simple, although that is a trend right now. Good luck! That's tough! I hope the room is big enough for all of them--but I have done 3 in a room before too (all girls though)!