V- in the case of the M3 (and many other virtual ground amplifiers) depends upon your perspective, but is actually the - of the single ended power supply. It functions on this schematic as the - rail, (or V-). Ground reference is generated by a TLE2426 and is 1/2 the potential of V+ and V-. So for example if you are using a 24Vdc power supply, if you measure V- in relation to the - lead of the PSU, you would get 0V. However, if you measure it in relation to the virtual ground (SG on the schematic), you will see -12Vdc for V- (and +12Vdc for V+).