The molecules in solid are regularly arranged. They are held together by very strong intermolecular forces of attraction that's why they cannot move from one place to another.
In liquid, the molecules are also close together but not as in solid. They are not regularly arranged. The particles can move freely from one place to another within the volume of the liquid.
The gas molecules are very very apart and the intermolecular forces of attraction are weak that's why the particles are very free to move from one place to another.
