One month post c-section have my period,until now for almost 3weeks,the first 3days it is heavy the remaining only just a spot,is it normal?

Most likely yes: Your first period can come as early as 6 weeks after delivery and may be heavier than your typical cycle. Also depending on your activity level, you may spot continually, upwards of 6 weeks, after delivery. You should start experiencing normal regular cycles within this next month. Remember to use contraception if sexually active even if breast feeding.