How many chambers does a frog heart have and why is this advancement necessary for a land animal?

A frog heart has 3 chambers, 1 ventricle and 2 atria. One atrium (the right) receives deoxygenated blood from the frog's body (in the systemic circulation) and then pumped to the single ventricle wherein it will go to the lungs to be oxygenated. Another atrium (the left) received oxygenated blood from the lungs then pumped to the single ventricle wherein it will go to the systemic circulation. In the single ventricle, some form of mixing of the oxygenated and deoxygenated blood happens but since the frog is a cold blooded animal, the decrease in the concentration of oxygen is deemed superficial. 

This advancement allows the frog to live on land using the lungs as the primary organ for oxygenation of the blood.