A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish). At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia (Greek ἀμφí, amphi, "both" + βíος, bios, "life"). They inhabit a wide variety of habitats with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Amphibians typically start out as larva living in water, but some species have developed behavioral adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely upon skin. They are superficially similar to reptiles but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
Reptiles the class Reptilia, are an evolutionary grade of animals, comprising today's turtles, crocodilians, snakes, lizards, and tuatara, as well as many extinct groups. A reptile is any amniote (a tetrapod whose egg has an additional membrane, originally to allow them to lay eggs on land) that is neither a mammal nor a bird. Living reptiles have scales or scutes (rather than fur or feathers) and are cold-blooded.
Wikipedia link - Fish
Wikipedia link - Amphibians
Wikipedia link - Reptiles