My son (now 5 months) has already had two fevers. I ran to his pediatrician each time. She told me a fever in a child (with no other symptoms) is not necessarily something to worry about unless it is:
-high (this will depend on child’s age. I believe for a 3 year old it would be anything over 103).
-lasts for longer than a day
-is accompanied by vomiting or another symptom.
If your child has not been immunized, it could be a symptom of a much larger and more serious issue. I would assume, however, (as a 3 year old) this is not the case. My son’s first fever was before any immunizations. So, he needed a series of tests (he hadn’t yet had the Pneumococcal vaccination or meningitis vaccination).
If you are concerned, you can always call your pediatrician. I call ALL the time, maybe it’s a first time parent thing. If you look online I recommend a “good” site, something like mayo clinic.
Good luck! I hope the little guy feels all better!