Instructions
Verify that your Nikon has a multiple exposure mode. Many cameras do not have this function and so it will be missing from your command menus. In this instance, you will not be able to combine three exposures without using an outside tool, such as Photoshop.
Press MENU, then select SHOOTING MENU. Scroll down to select the "Multiple Exposure" option. Press the right arrow and select the number of shots. Set this number to three to combine three exposures. Press the OK button.
Shoot your first exposure. You can review this exposure. Shoot the second and third exposures within 30 seconds; if there is a delay of more than 30 seconds, the camera will revert to normal mode.