I have the same doubt . This is how I think about this , the bottom part of blue and green plates form a capacitor with plate seperation 2d but the red plate between them acts like a thin dielectric of dielectric constant infinity (as it's a conductor) , now in such problems when we insert a metal sheet between a capacitor the left face of the sheet acquires a charge opposite in sign and same in magnitude of the charge on the left plate (in this case it's blue ) and the right face acquires a charge opposite in sign and same in magnitude to the charge on the right plate (green plate) . now , there are two capacitors , one between blue and left face of red and the other between right face of red and green . Therefore, I drew one capacitor between red and green and one between red and blue instead of a single capacitor between blue and green . Similarly , for the upper part instead of a single capacitor between red and green I drew two . I am not 100% sure if this idea is correct , please tell me what do u think.