Final answer:
The best initial treatment for a 3-year-old with a URI is supportive care since most URIs are caused by viruses, which do not respond to antibiotics or antivirals.
Step-by-step explanation:
The best initial treatment for a 3-year-old with an upper respiratory infection (URI) is supportive care. Upper respiratory infections are commonly caused by viruses, which means that antibiotics and antivirals are usually not effective as URIs are typically mild and self-limiting. Supportive care includes measures such as ensuring adequate fluid intake, rest, and controlling fever or discomfort with the appropriate doses of over-the-counter medications like acetaminophen or ibuprofen, under the guidance of a pediatrician. Overprescription of antibiotics for viral infections can contribute to antibiotic resistance and should be avoided unless a bacterial co-infection is confirmed by a healthcare professional.