When a 3year old has a fever what should I do?

Fever. Fever is the bodies way of fighting infection. Keep your child comfortable offer plenty of liquids and tyelenol or Ibuprofen to help keep the temperature down. Fever lasting longer than 5 days or fever > 104 would require immediate attention as would fever with headache and vomiting.
Cooling measures. Fever is not always a bad thing; some state a fever is just proof that your body is fighting off something. High fevers >105 need immediate cooling (tepid bath, cool wash cloths to the neck, armpits and groin). Fevers also generally respond to tylenol (acetaminophen) or Motrin as well. Fevers <105 should only be treated for comfort reasons, using similar treatment methods as above.