The three creatures are a baby on a mothers back, the mother carrying the baby on her back and a child inside the mothers stomach. The baby on the mother's back did not move itself but still entered the building because the mother carried the baby inside the building. The second creature was the mother because she could enter the building by moving. The third creature is a baby inside the mothers stomach, and it did not move itself and didn't know that it was in a building.
Today's quiz courtesy of Braingle.com.