Impetigo bullosa: This condition is caused by Staphylococcus aureus and is characterized by the formation of large bullae that can rupture, leaving a honey-colored crust. The presence of yellow fluid in the bullae supports this diagnosis, as it is consistent with the purulent nature of the infection.

Epidermolysis bullosa: A group of genetic conditions that result in blistering of the skin. While it can present at birth or shortly after, the presence of yellow fluid might not be as typical, but it cannot be ruled out without further investigation.
Staphylococcal scalded skin syndrome (SSSS): Caused by staphylococcal toxins that typically affect infants and young children, leading to widespread blistering and peeling of the skin. The yellow fluid could be consistent with a secondary infection.

Herpes simplex virus (HSV) infection: Although less common, HSV can cause severe disease in neonates, including skin, eye, and mouth lesions, and can progress to more serious forms like encephalitis or disseminated disease. The presence of vesicles or bullae, especially if they are painful, should prompt consideration of HSV.
Congenital syphilis: Can cause bullous lesions among other symptoms. Given the potential for severe outcomes if untreated, it's crucial not to miss this diagnosis.

Bullous pemphigoid: An autoimmune disease that can very rarely affect infants, characterized by the formation of bullae. It would be unusual in a 3-week-old infant but should be considered if other diagnoses are ruled out.
Acrodermatitis enteropathica: A rare genetic disorder affecting zinc absorption, which can lead to skin lesions, including bullae. The condition is very rare and would require specific diagnostic testing to confirm.