Bajoran wormhole schematic
The B3 was a curve in the Bajoran wormhole. The curve was located between the ring singularity and F1.
On the chart "The Tunnel in the Sky", which was seen in the schoolroom aboard Deep Space 9 in 2369, B3 was one of the components labeled on a map of the wormhole. (DS9: "In the Hands of the Prophets")