Since a frog has 3 chambers, why is is? And how can a frog regulate its body temperature?

Answer 1

A frog has a three chambered heart, with two atria and a single ventricle that pumps the blood. A consequence of this is that the oxygenated blood (coming from the lungs and skin) and deoxygenated blood (coming from the tissues) mix. This is less efficient than the 4 chambered heart of mammals, which keep oxygenated and deoxygenated blood completely separate.
Frogs are ectotherms; they cannot physiologically regulate their body temperature. Their body temperature is directly related to that of their environment; their temp goes down if it gets colder. The only thing they can do is behaviorally thermoregulate (kind of like us putting on a sweater when we feel cold); they might move to a place that is warmer, or bury themselves in the mud.