I have had my period for three day the first two where normal bit the last was very light and didnt last for long now im having this creamy thick wha?

Vaginal discharge. At 17, your personal normal pattern of menstruation, vaginal fluid amount, etc may still be evolving. If the white discharge clears in next 1-2 days and no other symptoms, maybe nothing need be done. If it persists, or if other symptoms (pain, itch, odor), get care ASAP. If sexually active, STDs may be a concern; get tested if in doubt. Australia's govt funded sexual health centres are superb.