I have a small hard lump in my neck its been there for 3 months, what could this be?

Neck lump. Any prolonged, suspicious lump should be examined by your physician. The list of potential differential diagnosis is quite extensive and best handled by an experienced provider.
Many things. Lumps in the neck may be cysts, enlarged salivary glands, enlarged lymph nodes or other tissue masses. The lump can be evaluated by ultrasound to see if it is solid or cystic. Lumps that do not resolve in a short period of time can be aspirated by fine needle aspiration very easily and any cells can be examined under the microscope to determine what is causing the lump.
Lump. It depends where the lump is in the neck and your age. If a child a midline neck mass can be a thyroglossal duct cyst. If off to the side it can be a skin cyst or lymph node. In adults, thyroglossal duct cysts are rarer both skin cysts and lymph nodes do occurr.