Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of non-productive cough and nasal congestion in an otherwise healthy adult. The symptoms presented, such as non-productive cough and nasal congestion for 3 days, are typical for a viral URTI, and the physical exam being normal further supports this diagnosis.

Allergic Rhinitis: This condition can cause similar symptoms, especially if the patient is exposed to allergens. The lack of other symptoms like sneezing or itchy eyes might make this less likely, but it's still a consideration.
Acute Sinusitis: Although the patient's symptoms are not severe, and the physical exam is normal, acute sinusitis could present with cough and nasal congestion, especially if the infection involves the sinuses.
Influenza: Depending on the time of year and the patient's vaccination status, influenza could be a consideration, especially if the cough is accompanied by systemic symptoms like fever or body aches, which are not mentioned but could be mild.

Pneumonia: Although the physical exam is normal, pneumonia can sometimes present with minimal findings early in the course. A non-productive cough could be an early sign, and missing this diagnosis could have serious consequences.
Pertussis (Whooping Cough): This bacterial infection can cause a persistent cough and is a significant concern due to its contagiousness and potential severity, especially in infants who might be exposed to the patient.
Tuberculosis (TB): A chronic cough could be a symptom of TB, and although it's less likely in an otherwise healthy adult without risk factors, it's a diagnosis that cannot be missed due to its public health implications and the need for early treatment.

Sarcoidosis: This autoimmune disease can cause a variety of respiratory symptoms, including cough, but it would be unusual for it to present so acutely and with such minimal findings on physical exam.
Interstitial Lung Disease: Similar to sarcoidosis, this would be an unusual presentation for interstitial lung disease, which typically has a more chronic course and other associated symptoms or findings on physical exam or imaging.
Lung Cancer: Although extremely rare in a 35-year-old without risk factors, any persistent or unexplained cough should prompt consideration of this diagnosis, especially if the patient has a history of smoking or exposure to carcinogens.