Your baby learnt to wake up every 3 hours because you taught this. He does not know what is right, or what is wrong, he only learns from you, and that is for him what is correct. If, every time he wakes up, you take him out of his cradle, you walk with him in your arms, or you take him to your bed, the baby will learn that this is correct, and every time he wakes up in the night, he will wait for this same “correct” response from you.