Normally 3cm you don't feel the intense pain yet but can ask for it. The epidural will be put to low dosage when you're around 9-10cm so you feel and push baby out. Not painful cos epidural makes you numb waist down but you'll feel pressure down there when baby's about to come out. Or you don't feel anything at all.
upon admitting into the hospital, express ur desire to have the epidural, so they can make the necessary arrangements. dont wait til its unbearable. the nurses will start off with low dose, and increase gradually