Im 20 and i found that i'll have my period a time in 3 weeks. when it reaches the 3rd day, flow is much reduced. it is the same the 4th day. is it normal?

That sounds ok: Some women have 4 day periods with a couple of heavier days and a couple of lighter days. If the periods come irregularly (not monthly), if they are painful, or if there are other symptoms, one can see her primary care doctor for an evaluation to check for any problems.

Menstrual Bleeding : Some women cycle every 3 weeks, others monthly. It is quite normal to experience a slowing of flow as the menses trail off if this represents a dramatic change in your cycle, you might want to let your gynecologist know and discuss possible causes.

Sounds normal : Periods come every 21-35 days and last a variable amount of time. A four day cycle that decreases over time sounds like a very normal period.