3D sounds likely have to be single channel (mono). This is because a multi-channel (stereo, surround etc) sound makes no sense in a 3D sound environment context.
Multiple sound channels are derived by using sound sources in a 3D sound environment with multiple sensors placed at distinct locations. For example, stereo sound usually assumes that the two sensors are placed about 1 human head width apart within the 3D sound environment so that what they capture mimics what a human would hear if they were physically present inside the 3D sound environment. As such the sound of an explosion to the right of a player in a virtual 3D game environment only needs a single channel, with the correct stereo output being produced so that the player's right ear hears the explosion more loudly than their left ear.
Of course 3D sound environments can be very complex with directional components, phase changes, distortions and even interference. However in the case of games, especially older ones, it is often kept quite simple. I do not think WC3 even simulates Doppler effect with its 3D sound model, something a game like Freelancer did (although it made no sense given the games context) before Warcraft III was released.
