Thank you.Stock Custom Shop historic reissue (57 LP, 61 SG), vintage (unless modified) scheme: middle pickup 'piggybacks' onto or is 'paired' with the bridge pickup and controls; only active with toggle in the middle position. Think 'bridge-plus' versus 'neck-plus-bridge' in middle, while up and down are neck and bridge respectively. When in middle, controls blend the middle in, so turning down the volume changes the blend to lessen the bridge more than the middle, such that the diddle shouldn't go fully silent when bridge volume is zeroed.
-- If Modern USA e.g. new Supreme models: 3 individual volume knobs (one per pickup: neck, middle, bridge) + one master tone knob + 3-way toggle switch. All 3 volumes have push/pull pots for coil-tap ("splitting" to pseudo-single-coil). Master tone operates regardless of pickups, a-la "Mickey Baker"-style but with the push-pull pots.
-- If Vintage/Reissue modified: all bets off, who knows...?