ok the honest answer is that being 3 cm indicates you have started labor but no indication of when your labor will stop. I went into pre-term labor and asked the question to many doctors and nurses because I wanted to try to hold labor off as long as possible. I was given the answer I could deliver that day or in months. My obgyn said she had a patient that lasted 3 months at 4 cm. Do you know how effaced you are? My doctor told me that has more to do with how soon labor will begin, but still not a sure thing. I was 90% effaced and 2 1/2 cm and lasted a week, was told the average would be 2 weeks but it is different for everyone. 100% effaced is when the cervix is completely thinned out so a general rule is if you are close to complete labor could be very soon and if you are at the very early stages, 0-50 it could be a while. But people have went from zero to 100 in a matter of hours, same thing applies to the position of the baby. There are stations and depending on how far in desent your baby is could indicate how much longer you have to go but also again it can vary. The only thing I can say for how you will know you are in active labor is most people really really know, there is no denying it, they are in a lot of pain. Although there are cases where people with high pain tolerences don't realize they are in labor until very close to delivery. You will know if your water breaks, or if the pain increases to something that you have to concentrate on is the best way to describe it. It makes you stop what you are doing, you have to try your best to do things to distract yourself from the pain, go to the hospital. The rule of thumb the dr's give you is if you are earlier than 37 weeks and your contractions are more than 4 in an hour, go to the hospital (this is a sign of pre-term labor) if you are after 37 weeks, it's ok if your labor begins (no reason for them to intervene and try to stop it) and you can wait until you're contractions are about 4-5 min apart before you need to go. Use common sense though, if your are far from the hospital, leave your home sooner, go to dr's, if you are sent home, go somewhere nearby, if noone lives near, even a motel so you are close if labor progresses quickly. Make your hospital bag. A couple outfits for yourself, an outfit for the baby with a hat, socks, 6 or 7 receiving blankets, they will give you diapers/ etc. (maybe even 2 outfits, one smaller and one larger in case you have a large baby.) If you have any bleeding that is red, go to the hospital as well. If you think you are leaking any fluid, also go to the hospital.
also i forgot, put the carseat in your car. pack toothbrush, toothpaste, a hair brush, shampoo, conditioner, a nursing bra, regular full underwear (this seems basic but easily forgotten if you pack during labor, you will have other things on your mind). ( and time lower back pain as well, labor can occur in abdomen, in back, or both.