I was late for 3 days on my period and today i had spotting pinkish brown. am i having my period or somethings happen cause this is the first time having this spotting pinkish brown color?

This may well be simply the start of your period but you need to wait to see how this evolves. Menstrual flow can vary a lot and your past experience is not predictive. Your cycles and periods can be affected by many things (not just pregnancy) and if you continue to be late and/or have only spotting then it would be reasonable, though not urgent, to do a pregnancy test and see a doctor.