On my BN2, the lap belt anchors are through the floor, rear corners, behind the seats. Large thick reinforcing washers under the floor. About 3" x 3" X 3/16 thick. You will have to look around underneath to find a clear space in the floor for exact location. Preferably near but not over a structural member. I don't have any idea where you would put a third point for a shoulder belt. There doesn't really seem to be a good place. Later Healeys had factory provided anchor points for three point mounting, but they are an entirely different inside layout than the Hundreds.
D