What can be the cause of liver inflammation where hepatitis a / b / c is negetive?

Lots. Can be due to toxins like alcohol, fat deposition from drugs, overweight, diabetes, meds, autoimmune, genetic abnormalities of metabolism, e.g. Iron or copper, other viruses like epstein-barr and cytomegalovirus (both cause infectious mono), and others.
Further testing. There are other reasons such as primary sclerosing cholangitis, disseminated histoplasmosis, auto-immune hepatitis, and others. A liver biopsy may be indicated. Followup with a hepatologist.