Why three conductors for an audio signal? Here are some answers:

Grounding/shielding: With the third conductor put to ground, together with the two shields, we have increased RF-rejection to 115dB (20dB better than standard) at 25 kHz. This fact has also been conformed at the AES-paper held by Mr Neil A. Muncy ("Noise Susceptibility in Analog + Digital Signal Processing Systems") in November 1994. The GAC-3 was named the best performing microphone cable available.

Round Construction: 3-conductor constructions are round constructions, and since the cable has the freedom to move in all directions, especially on the strain relief of a connector, the cable will survive more movement cycles.

Phantom Power: The 3rd conductor can be wired as a drain wire for a reliable connection of phantom power to the microphone without affecting the shield.

Each of the three conductors consists of 96 (!!) strands of 0.05mm copper wires being the finest stranding for audio cables available which gives you improved flexibility, better signal transport and longer lifetime (moving cycles). (Heavy duty version with 5.8mm Ø also available)

Paper from "Neil A Muncy" (AES Member)
Noise Susceptibilty in Analog and Digital Signal Processing Systems Presented at 97th AES Convention San Francisco November 1994 Revised April 13, 1995 Journal of the Audio Engineering Society Vol 43 #6 June 1995 p.435-453