I have 3 blue spots under my skin one on my arm on outer thigh and on buttock they are sore when pressed on been there for years what could they be?

Many things. Acute or sudden blue painful spots can be due to bruising. They can come from systemic issues ( like liver, etc), hemaniomas and other things both "bad and not so bad". I would get a further evaluation with more testing . I woul imagine that the fact they have been there for a long time suggests the probably represent nothing serious... Not i would still get them checked out.