I attached an image which shows a point to point wiring diagram for the wires. The wires between the ignition module (mounted inside the distributor on the breaker plate) and the top mounted coil are the (2) coil primary leads. The 2 coil primary leads are essential. I know there are 3 wires as you described but its been decades since I wired an HEI, the attached wiring diagrams do show a ground wire, so it may be a ground wire.