Can I sell my house which has a Mechanic Lien not enforced after 3 months ? If not ,what should I do ?

You have a stale lien issue. You can always sell the house but a title insurer might not write insurance on it unless the lien is removed.  If you are working with a realtor ask him or her to contact a title company, get a preliminary title report, and see whether the insurer would provide insurance for the title with a stale lien on title.  It may be necessary to speak with a title officer.  If there is an issue, you must give the contractor a written notice to remove the lien in a format and sent in a manner that is specified in the Civil Code.  If the contractor refuses to release the lien within ten days after notice is deemed received you can file a petition to expunge the lien and recover attorneys fees and costs from the contractor if you win.  I can handle this issue for you.  It is a single hearing that is supposed to be heard within 30 days of case filing.  There are some procedural and jurisdictional requirements to comply with before the case can be filed. 

You can list your home for sale and the real estate agent will get a listing preliminary title report to see whether the mechanic's lien has been removed for failure to follow up with a lawsuit.