Yeah, it would be nice if they added to the product page something that says an ATX power supply with a -5V line is needed. Internally the ATX power supplies have the -5V available, they just don't run the line to the connector any longer as PC motherboards don't use -5V.
A couple things I would try. Test the pin voltages on the adapter to see what ones match the A3000 power spec by comparing to the original A3000 power supply connector voltages. Especially what comes out of the -5V pin on the adpater.
Amiga pin # Description
1 +VID (this should be +5V)
2 +5V
3 +5V
4 +5V
5 No connect (or IIRC the A3000 PSU has +5V here, at least it was on mine)
6 Gnd
7 Gnd
8 Gnd
9 Gnd
10 Gnd
11 -5V
12 +5V_USER
13 TICK
14 -12V
15 +12V