Yes, it is possible to apply Vaseline on the scalp of a 3-month-old child. However, it may be a bit more difficult to clean compared to other options like olive oil. It is recommended to try a small amount of Vaseline on a small part of the affected area to see how it works. Generally, cradle cap is not a dangerous condition and tends to resolve on its own over time. If you have any other questions, feel free to ask for further assistance.