Fever for three days with nausea, headache, joint pain, loss of appetite, and weakness is often due to a viral infection, such as the flu or sometimes infections like dengue or other seasonal illnesses. These conditions can cause body aches, low energy, and fever. Take rest, drink plenty of fluids, and eat light food to support recovery. As the fever has lasted for a few days and has multiple symptoms, consult a doctor to confirm the exact cause and treat it properly.