For the 2013 Audi A3, both the low-beam and high-beam headlights use D3S bulbs. The daytime running light and parking-light fitment depends on the headlight configuration: models with halogen headlights use a 5201 daytime running light and either a 2825 or 98 parking light, while models with HID headlights use an LED daytime running light and a 2827 parking light.
Fog-light fitment is listed as either H11 LED or H16 LED, so verify the existing lamp configuration before ordering. The front side-marker bulb follows the same headlight-based distinction as the parking light: 2825 or 98 with halogen headlights, and 2827 with HID headlights.
Useful rear and exterior replacements include a 921 reverse light, 7506R brake and tail lights, a 7507 rear turn signal, and a 6418 license-plate bulb. The center stop light is LED. Interior applications include 6418 for the courtesy and vanity lights, 6411 or 6428 for the dome light, 64111 for the map light, and 2825 or 98 for the glove-box and trunk lights.