How bad a fever before i take my 3 year old to the er?

And what else???: A 3yo is likely to have many illnesses /yr due to simple viruses & most will be associated with mild to moderate fever. If the kid is playful most of the time & taking fluids well enough to pee every 6 hours you may be able to handle it at home. If the kid acts tired & whiny most of the time or begins to refuse fluids, the kid should be examined.