You can enable A3D in gta_vc.set.

First, enable Aureal A3D in GTA3. In my example it's A3D 2.0. Then:

!!!back up gta_vc.set just in case!!!

Easiest way:

1. Copy Documents\GTA3 User Files\GTA3.set as Documents\GTA Vice City User Files\gta_vc.set
2. Re-adjust settings

More correct, somewhat harder way

1. Open Documents\GTA Vice City User Files\gta_vc.set in a hex editor
2. Find out which part is modified when you change audio hardware (it should be after "...morestuff...")
3. Find same part in GTA3.set (with A3D selected) and replace the data in gta_vc.set with values from GTA3.set

!!!Check other settings when done, especially -- Radio station, Speakers configuration, Dynamic acoustic modeling, Brightness -- they may be affected because their bytes are located nearby: https://gtamods.com/wiki/Gta_vc.set
!!!Do not modify Audio hardware, or A3d will disappear and you'll have to start over!!!

Proof: