Having a cold, fever, and nausea for 3 days is usually due to a viral infection, which often improves on its own with proper care. The main focus should be on rest, staying well hydrated with water, coconut water, or oral rehydration fluids, and eating light, easy-to-digest foods like rice, soups, or fruits. For fever and body discomfort, you can take Paracetamol as per the recommended dose, which is generally safe for most people. For cold symptoms, steam inhalation and warm fluids can help relieve congestion, and simple anti-allergy medicines may be used if advised by a doctor. For nausea, eat small, frequent meals, avoid oily or spicy food, and ginger tea or medicines can help reduce the feeling. Most viral illnesses settle within a few days, but if your fever continues beyond 4-5 days, becomes very high, or if you develop severe vomiting, dehydration, breathing difficulty, or weakness, you should consult a doctor for further evaluation.