You mentioned that your daughter is only 3 years old! She is too young to know why it is happening to her so please take her to the doctor before it gets worse. There could be several reasons for the problem you have described. Do not try to diagnose and treat it yourself. Some possible reasons are listed below, but you will need a doctor's diagnosis to determine which of them is the actual cause.
Bacterial infection of the vagina is very common in girls between the ages of two and four years old. This happens when bacteria from the surrounding skin enter the vagina and multiply. Good hygiene can help prevent this problem. Always wipe her private area from front to back instead of back to front to avoid any infection from fecal matter. Encourage your child to wash her hands with soap. If she frequently touches/scratches her genital area with her hands, then dirty hands will increase the risk of a bacterial infection, which may eventually be the cause of pain.
Use only mild, baby soaps for your daughter, as harsh soaps or body washes may cause rashes and skin irritation. Also make sure that you wash her private area thoroughly so that no soap remains, as soap residue causes skin irritation and a baby's tender skin reacts to it quite easily. At the age of 3, the natural response is to scratch an itch. The pain could be because the child has been scratching too much in the area.
Make sure your daughter wears clean, comfortable underwear and clothing. Use natural fabrics like pure cotton, which will allow her skin to breathe. Most stores now have clothes made of special extra soft cotton fabrics specially designed for kids.
You must also realize that child abuse is something you cannot ignore or rule out. Take care and see to it that your daughter is not left alone with people who you don't know very well. Little kids do not understand sexual abuse and are prime targets of molestation. If you have left her alone at a neighbor's house or somewhere else, if there is a change in her behavior and the symptoms you mentioned have started after this, you need to take her to the doctor immediately.
Your doctor will probably run a few tests and tell you the reason for it. It is very important that your child is examined and treated correctly. Please do not be afraid or embarrassed of taking her to the doctor.
answered by G M