The brief
The A3 had a cluster of symptoms going at once: oil disappearing faster than it should, a rough idle, misfires, a flat spot and a general loss of power. He brought it in. That points at the oil separator and its valve in the crankcase breather. As the engine runs, oily mist builds up inside the crankcase, and the breather system vents that back into the intake to be burned cleanly, with the separator catching the oil and the valve regulating the flow and the pressure. When the separator fails, raw oil gets pulled into the intake and burned, that's the oil consumption, and when the valve goes the crankcase pressure isn't controlled, which roughens the idle, causes misfires and saps power. A failed separator and valve don't recover, so they need replacing.
The diagnosis
Diagnostics confirmed it, fault codes for the crankcase ventilation system and a lean mixture with misfire counts logged, and a check showed the separator wasn't holding pressure and the valve wasn't regulating it, pulling oil through the breather. The intake, the injectors and the rest of the engine checked out, the oil burning was coming through the breather. That's a replacement of the oil separator and its valve, sealed components you don't rebuild on the car, so the call was the complete units.
The work
The old oil separator and valve were removed and new genuine Audi-spec parts fitted with fresh seals, the breather hoses checked over while everything was apart. Then the fault codes were cleared and the engine's fuelling adaptations reset so it could relearn against a breather system that was working. A road test confirmed a steady idle, no misfires, no flat spot, and the power back.
The outcome
Steady idle, clean response, no misfires, no warning light, and the oil consumption back to normal. The A3 went home running properly again. A failed oil separator and valve quietly feed the engine its own oil and keep the running unhappy the longer they're left, so changing both and letting the engine relearn put it right.