Hello Doctor I'm feeling mild fever past 3 days...

what are the causes of  fever
in general there are many causes of fever but we will mention the most common ones
any body infection
chronic or acute siusitis
earwax
otitis media and mastoiditis
hypertention
convulsion
brain abcess
brain vasculitis
treatment of  fever
the treatment according to the cause but the most common cause to these symptoms is acut upper  respiratory infection or Urinary tract infection 
and you must take medication like pain killer like paracetamol 500 mg tab 3times in day and also antihistamin like loratidine one tab 2 times for aday with good nutrition and if need getting enough
 eat fruit and vegetbels
or if there is Urinary tract infection: Antibiotics such as Ciprofloxacin 500 twice daily for 7 day
Investigations to do
You can ask for CBC to see the level of WBCs which increased in inflammation and infections of ears or lungs or urinary tract or birian like miningitis . Also ,you can ask your internal  doctor for examination of upper respiratorry tract and tonsilles or renal 
and if you with direct contact with infleunza pt you must do tr-pcr for corona virus
my advise
We recommend that you go to the internal doctor if symptoms increase to keep you safe