How do I calculate day 3 of my menstrual cycle?

Day 3 of your menstrual cycle is the 3rd day after your period has started. If your period starts late in the day or you are only spotting, start counting from the following day when you have proper blood flow. Don't worry about being a day or two out although we advise day 3 testing, samples taken on days 2, 3, 4 or 5 are also fine.