Which is the difference between the three mammal groups?

The three main groups of mammals are monotremes, marsupials, and placental mammals. Monotremes, like the platypus, lay eggs and produce milk but lack nipples. Marsupials, such as kangaroos, give birth to underdeveloped young that continue to develop in a pouch. In contrast, placental mammals, including humans, have a complex placenta that nourishes the developing fetus during a longer gestation period inside the womb.