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The swelling seems have been a hematoma
(collection of blood). It takes a while to absorb. But never three years. It may have got calcified and hence the swelling is persistent. If that is the case then it can be removed. But remember that for removing the hematoma, the surgeon will have to a give an incision and that will leave a scar. Although with modern fine sutures, it will be a very thin barely noticeable line, but there will a scar. You should consult a pediatric surgeon or plastic surgeon for the same. Alternatively, you can give the swelling some more time. It may regress further. But is important to consult a surgeon to get a definite diagnosis. So that we are not missing anything.
Hope that helped
God bless you