Acute Bronchitis: This is the most likely diagnosis given the symptoms of cough with expectoration for 3 days. Acute bronchitis is a common condition characterized by inflammation of the bronchial tubes, often caused by a viral infection, and typically presents with cough, sometimes producing mucus, and possibly accompanied by other symptoms like sore throat or fever.

Upper Respiratory Tract Infection (URTI): URTIs, such as the common cold, can cause cough and expectoration due to postnasal drip. Although the primary symptoms are usually centered around the nose and throat, cough can be a significant symptom.
Pneumonia: While less common than acute bronchitis or URTI, pneumonia is a possible cause of cough with expectoration, especially if the sputum is colored or if the patient has other symptoms like high fever, chest pain, or difficulty breathing.
Asthma: Asthma can cause cough, especially at night or with activity, and may produce mucus. If the cough is persistent and accompanied by wheezing or shortness of breath, asthma should be considered.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Although less likely in a 35-year-old without a history of smoking or exposure to lung irritants, an exacerbation of COPD could present with increased cough and sputum production.

Pulmonary Embolism: Although cough can be a symptom of pulmonary embolism, it is often accompanied by other more alarming symptoms like sudden onset of chest pain or shortness of breath. It's crucial to consider this diagnosis due to its high mortality rate if untreated.
Tuberculosis (TB): TB can present with chronic cough, sometimes with expectoration, and is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.
Lung Cancer: While rare in a 35-year-old, lung cancer can cause cough and should be considered, especially if the cough is persistent, does not improve with treatment, or is accompanied by other symptoms like weight loss or hemoptysis.

Cystic Fibrosis: Typically diagnosed in childhood, cystic fibrosis can cause chronic cough and sputum production. However, it would be rare for this condition to first present in a 35-year-old without any prior symptoms.
Bronchiectasis: This condition involves permanent enlargement of parts of the airways and can cause chronic cough and sputum production. It might be considered if the patient has a history of recurrent infections or other underlying conditions.
Allergic Bronchopulmonary Aspergillosis (ABPA): This is a rare condition that occurs in people with asthma or cystic fibrosis, caused by an allergic reaction to a fungus called Aspergillus. It can cause cough, wheezing, and production of brownish plugs or flecks in the sputum.