darkness under the eye lid in babies is seen because the baby's skin is thinner than normal due to which the blood vessels underneath it makes the area appear darker. This can be only understood when you see the baby in person so it good to visit the doctor and get a clarity on the same 

Hello mala. Darkening under the eyes of a 3 yr old boy could be due to exposure to mobile or less sleep. Even if the child is very active , we have to ensure they have adequate sleep. This can be assumed to be normal but you have to ensure you keep him engaged in some other form. I also believe it might take some time to reduce this darkness under the eyes.