The gas diffuse fastest and solid slowest.
Why : There are two reasons to it
One the intermolecular force of attraction in gases is very less in comparison to liquids and solid. This is maximum in case of solids.
The other is very high kinetic energy of gas molecules than particles of solid. This again is due to low intermolecular forces and large space between two particles.
Thus gas diffuse faster than liquids and later diffuse faster than solids.