Is a fever of 99.3 for a 3 month old, too high?

Not necessarily.: Gold standard is a rectal temperature of 100.4 or greater for the definition of "fever" in infants. Fever is merely a symptom and sometimes time will determine how the thermostat of your infant will behave-better/worse. 99.3 could be a normal variation or a sign of an illness. Other symptoms associated to it will remain important: intake, output-wet/dirty diapers, could sx's, behavior, sleep.