Amphibians
Amphibians have a three-chambered heart - two atria and one ventricle. The mixing of oxygenated and deoxygenated blood is kept to a minimum due to the timing of the contractions between the atria. This is called an incomplete double circulatory system.
Amphibian lungs are balloon-like structures where gas exchange is limited. As a result, oxygen can be diffused through their moist skin (and sometimes their mouths) to compensate.
Amphibians force the air into their lungs using throat muscles in a process called buccal pumping.
Reptiles
Reptiles have a three-chambered heart – two atria and one partially divided ventricle. There is a mixing of oxygenated and deoxygenated blood because the ventricle is not split completely.
The only exception to this physiological structure in reptiles is the crocodile. The crocodile has a four-chambered heart similar to mammals and birds where the ventricles are split completely.
Fish
Fish have the simplest vertebrate heart with only two chambers - one atrium and one ventricle. This is called a single cirulatory system.