They have more secrets.
A lot of factors in a third grader’s life lead to secrets. They are getting better at watching what they say and deciding when not to speak. They have active social lives in which they care deeply what others think of them but are afraid to be judged. They are withdrawing from adults, who they may have told secrets to previously. It’s no coincidence that this is a time when children end up holding certain information to themselves.
What you should do: Buy your child a diary and/or a box that he can lock. Not only does this give him an outlet to express his thoughts, but a locked box is a small symbol of responsibility that he’ll appreciate.