Let us understand the meaning of the three terms listed in the above question. Hydrophytes are aquatic plants, xerophytes are plants that can survive in very dry environments, and mesophytes are plants that are adapted neither to a particularly wet or dry evironment. 

Let us now see the major differences in plants belonging to these three categories -

Xerophytes are known for having stomata that are fewer in number, sunken, waxy, and/or open only at night. They tend to have succulent leafs or stems that can hold more water, and also a deeper or wider root system.

Hydrophytes often lack cuticles, because there is no need to prevent water loss. Stomata are larger in quantity and typically open. The structure of the plant is less rigid because the plant is supported by water pressure. Roots are smaller, because water can diffuse directly into the leaves. Surface-dwelling hydrophytes have large, flat leaves to aid in floatation. Air sacs may also be present for floatation.

Mesophytes lack special adaptatations to extremes of climate, leaving them susceptible to droughts and to waterlogging. They tend to have a certain degree of control over transpiration rates, to accomodate varying levels of moisture in the environment.