As the posters have said above, at age one you often see forward posturing of the mandible. 1st primary molars are flat so there is no real occlusal lock so kids posture all over the place, wherever is comfortable, not necessarily how the jaws relate. Typically at 3 years of age the relationship of the maxilla to the mandible is well established and you don't see a significant change. So, if your child has a class III tendancy at 3 I would be more concerned, but there are so many variables that there is no reason to get yourself worked up! Relax and enjoy your baby!
Emily