Bolt +3 is a heavily enchanted bolt, which is available in Baldur's Gate II: Throne of Bhaal. It has +3 bonus to THAC0 and it inflicts 1d8 points of missile damage. These bolts are sold at the Tankard Tree and by the merchant in Forest North.
In-game description
| “ | These enchanted missiles far surpass the craftsmanship of any bolt, magical or otherwise, you have ever seen. | ” |