I have a pain in my left groin, left buttock and it is painful to walk. Could it be a pinched nerve? It hurts to sit.

Yes, Pinched nerve. The description of the pain does fit a 'pinched lumbar nerve'. A lumbar spine MRI is the best test to check out the possiblility. You should get seen and you may get a prescription for Medrol (methylprednisolone) pack to help with the nerve inflammation. Warning signs are bladder/bowel changes and/or severe weakness. Hope you are better soon!
Perhaps. Groin and buttock pain and increased pain with standing/walking is most often related to a condition of the hip. The second most common cause of pain like you describe would be nerve root irritation in the lumbar spine. A visit to an orthopaedic surgeon for a history and physical exam will probably provide you an answer, but additional tests may also be warranted.