Solid can diffuse into liquid. When sugar is added to water, the whole water becomes sweet without stirring it because of the diffusion of sugar into the water.
One liquid can be diffused into another liquid as particles of both liquids are in random motion and will become equal throughout. An example is an alcohol in water.
Gas can be diffused in a liquid state. Carbon dioxide and oxygen are the two gases in the air which dissolve in water by diffusion.