To produce significant diffraction at the aperture need to be approximately the same size as the wavelength of the source, the light needs to spread out by diffraction before encountering the two other slits. The two slits act as coherent sources, because the light starts from the same source and is in phase at the double slit. Fringes are produced by the interference of the two waves propagating from the slits, the superposition of the two waves causes contructive and destructive interference where the waves are in phase and antiphase. I'm not sure if that's a good answer.