Benign pulmonary nodules: Given the small size of the nodules (6 mm, 5 mm, and 3 mm) and the patient's age, benign lesions such as hamartomas, granulomas, or fibrotic nodules are common and likely explanations. The pleural-based location can be seen in both benign and malignant processes, but the small size leans towards a benign etiology.

Metastatic disease: In an 85-year-old patient, the possibility of a primary malignancy elsewhere with metastasis to the lungs cannot be ignored, even though the nodules are small. Metastases can present as multiple small nodules.
Primary lung cancer: Although less likely given the small size of the nodules, primary lung cancer, especially if the patient has a history of smoking or other risk factors, needs to be considered. The pleural-based location of one nodule raises concern for a malignant process.
Inflammatory or infectious nodules: These could be due to a variety of causes including fungal infections or inflammatory processes, which can present as small nodules in the lungs.

Malignant pulmonary nodules: Regardless of size, any nodule has the potential to be malignant. Missing a diagnosis of lung cancer or metastatic disease could have significant implications for treatment and prognosis.
Pulmonary embolism (if the clinical context suggests it): Although the description provided focuses on nodules, in the appropriate clinical context (e.g., symptoms of chest pain, shortness of breath), pulmonary embolism could be considered, especially if there are other suggestive findings on imaging.

Lymphoma: Although less common, lymphoma can present with pulmonary nodules. This would be an unusual presentation but should be considered in the differential diagnosis, especially if there are systemic symptoms or other suggestive findings.
Vascular malformations or arteriovenous malformations (AVMs): These could present as small, pleural-based nodules but are less common and would typically have specific imaging characteristics.
Pulmonary amyloidosis: This condition can cause nodular lesions in the lungs but is rare and usually associated with systemic disease.