My 3 yrs old daugther have a hard lump behind of her left ear she have that since she was 3 months old..

Lump: If that lump has been behind her ear since she is 3 months old, it is either a cyst, a small lymph node, a bony piece of her scalp. It sounds like it is not of concern.

Lymph node: THis is most likely a retroauricular lymph node. It is typically benign. It is a result of an immune response of mild scalp abrasions. If it increases rapidly in size..(.it would have done so by now) it is a concern. Next time when you go in have it seen by your pediatrician.

Post-auricular node: There a lymph node behind the ear called post-auricular lymph node and sometimes it gets larger with minor infections of the scalp. sometimes it remains palpable (you can feel it) for a long time. As long as its small (