Dear Student,
FLUID: a substance that has no fixed shape and yields easily to external pressure; a gas or (especially) a liquid.
LIQUID: a substance that flows freely but is of constant volume, having a consistency like that of water or oil.
GAS: a substance or matter in a state in which it will expand freely to fill the whole of a container, having no fixed shape (unlike a solid) and no fixed volume (unlike a liquid).
Regards