Why dark brown or black colour blood discharge in small amount for 3 days at the time of mensuration?is there any problem with me?

No: Brown or black colored discharge is old blood... Meaning that it may have been blood from days or even weeks ago. If that's all you had, meaning you didn't have a true bleed, you may want to check a pregnancy test if you are sexually active. Otherwise, this light period with dark blood is solely due to hormones. If bothers you, can consider birth control pills to regulate. Good luck!