Yep, PS2 GH3 only supports what I call the "SG-style" controllers, that plug into the PS2 controller ports. They only added support for RB Strats and other USB dongle-based controllers starting with GH:A.
I think it's hilarious that, given the old controller compatibility debacle, the games with the broadest controller support are the Activision games on the PS2 (for fuck's sake you can even use 360 controllers on the PS2 games, I've successfully used my 360 RB1 drumkit with GH:WT on).
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