Since your diarrhea has lasted for three weeks and hasn't responded to Norfloxacin and Metronidazole, you should not take further antibiotics without a doctor's guidance. This persistent diarrhea and cramping could indicate a non-bacterial issue, such as Inflammatory Bowel Disease (IBD), Irritable Bowel Syndrome (IBS), food intolerances, or a resistant infection. You need to consult a gastroenterologist for a proper evaluation, which may include stool tests. For now, focus on staying hydrated using Oral Rehydration Salts (ORS), eat a bland diet, and avoid dairy and spicy foods. Please seek professional medical care to find the root cause.