My mom used tampons which kind of seemed pretty creapy to me as a little girl. However, she explained that when you grow up, women bleed and it doesn't hurt like bleeding from a scraped up knee or elbow or anything. It's not the same as an "owie" or anything to be afraid of.
I had a hysterectomy when my son was a year old so he learned about periods and mini-pads from having a sister 10 years older. We just kind of explained it as a natural course of events. Mommy didn't have periods because I didn't have any of my "reproductive" parts anymore. My daughter grew up knowing I was in the hospital all the time due to reproductive issues so she knew about uteruses, fallopian tubes, ovaries and endometriosis.
I think if you just explain things, as age appropriately as possible, kids are fine with it. You never need to give too much adult information, especially to a 3 year old. Your little one just needs to know you're not sick, it doesn't hurt, and it's a natural thing that happens to women's bodies.
Just my opinion.