Question

why there is hard to chamber or three chambered in some animals

Answer 1

As the two chamberd heart animals do not get blood fully pure as the deoxygenated blood get mixed with the oxygenated and the vice versa similarily this happens in the case of three chambered heart. But in case of four chambered heart the deoxygenated blood does not mixes with the oxygenated blood .Thus it is hard for two or three chambered heart.