Why do amphibians and reptiles have 3 chambered hearts?

Answer

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Hint: As we know amphibians are ectothermic and tetrapod vertebrates of the class Amphibia. They inhabit a variety of habitats. Whereas, reptiles are tetrapod animals that comprise all vertebrates which encase their embryos in a series of protective sacs except synapsids and birds. They are the creatures that either have four limbs or are like snakes which are descended from the four-limbed ancestors.

Complete answer:
Mammals and birds have high metabolic rate so they can deliver more oxygen per litre of blood to the body than amphibians.

A heart with three chambers is adapted for the needs of amphibians who could also absorb oxygen through their skin when it is moist. Except for crocodiles, which possess a four-chambered heart, all reptiles have a three-chambered heart that comprises two atria and one ventricle.

The chamber called the right atrium receives the deoxygenated, or also called as the spent blood which is returning from the body tissues. From there, blood is being conducted to the ventricle and is then pumped to the lungs. After picking up oxygen and removing carbon dioxide in the lungs, blood is returned to the heart and empties into the left atrium. The blood then enters the ventricle for the second time and it is pumped out of the body. The second trip through the heart keeps the blood pressure strong and blood flow is rapid as the blood is pumped to the tissues, thereby helping the blood deliver oxygen more efficiently.

Note: Three-chambered heart is considered to be a congenital abnormality in which there might be a single atrium with either two ventricles or a single ventricle with two atria. Rudimentary parts of the atrial and ventricular septa might be present but are incompetent in order to prevent a virtual single chamber in both the case.