I'm 23 yrs old. 3 week late period then light bleeding for 3 days. Negative pregnancy test the week before. Can it still be a possible pregnancy?

Unlikely. It sounds like you experienced an "anovulatory" cycle. When ovulation occurs, your cycle predictably happens about 14 days later. When it does not occur or is delayed, then you get a delay in your period, irregular bleeding and negative pregnancy tests. This can all be confusing. This should be discussed with your ob/gyn.