Why do frogs have 3 chambered hearts?

Though they breathe air using their lungs on land, they breathe through their skin using capillaries near their skin under water. When breathing through their lungs, ideally the blood entering the lungs to have CO2, so that it can be diffused out and O2 can be diffused in. When breathing through their skin, ideally the blood in the body has some CO2 so that it can be released through the skin, and O2 can be absorbed. In order to enable the frog to have CO2 in the blood in both the capillaries and lungs, it mixes the blood so that whether it's breathing through its lungs or its skin, it can manage either method passably. Although it is not specialized in breathing either on land or underwater, it is adapted to accomplish both passably, which is part of what makes it amphibian.