The -5V is only required for certain expansion cards, though I've never seen any cards that use it. The A3000 will run fine without it, the entire machine runs on +5V only.
+12V and -12V are required for the op-amps in the analogue audio stage and RS232 level shifters.
Connect +VID (pin 1 on the socket) to +5V. Unless the ATX power supply delivers the 50Hz tick signal, you'll need to move J350 to position 2-3 to source this signal from the on board generated VSYNC signal.
