Your 3-year-old grandson is certainly not too young to be diagnosed with autism if he has it. I would strongly encourage you to take him to his pediatrician (either you go with your son or alone.) Tell the doctor what he is doing and when.
Be prepared to answer questions about his sleep, eating habits, ability to connect with you or his father. The doctor will ask you many questions and offer help.
Head-banging is not unusual and by itself is not necessarily a sign of autism. Usually, though, children who head bang do so when they are younger. And slapping himself in the face is not a good sign either.
These could be made worse because his mother passed but I doubt that her death is the only cause.
Thank you for caring for your grandsons and for stepping up to the plate the way you have. You will undoubtedly make an enormous difference in their lives as they get older.