Option 1: This statement is true for mammals. Mammals are characterized by having hair or fur on their body. Hair is a defining characteristic of mammals.
Option 2: This statement is true for some mammals. There are mammals, like platypus and echidna, that lay eggs. However, most mammals give birth to live young.
Option 3: This statement is not true for mammals. Mammals have a four-chambered heart, which is different from the three-chambered heart found in reptiles and amphibians.
Option 4: This statement is true for some mammals. There are aquatic mammals, like dolphins, whales, and seals, that spend most of their lives in water.
In conclusion, the statement that is not true for mammals is option 3, which states that their heart is three-chambered. Mammals have a four-chambered heart.