I have a 3mm kidney stone just found by ultra sound. i do not have any back pain but i do have groin pain on the right side. how long to pass a stone?

Days to weeks: Do you have blood in your urine? Is the pain moving toward bladder?

Position: It all depends on the position of the stone. Given that you had an ultrasound exam i suspect the stone was found in a nonobstructing position in your kidney. These stones (at the size you mentioned especially) are rarely painful when located in the kidney. Ct scan would likely be necessary to determine if a stone is present in the ureter.