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Hi Music1969. You've got it!Is a simpler way to describe it @AndrewMason :
3-wire TRS cable is the typical Y-configuration headphones cable, where you have a single cable from the TRS plug and then at some point along the length of the cable, this splits off into 1 cable to each ear.
4-wire TRS cable is a V-configuration, where L-ground and R-ground split off from the TRS plug itself, running separately all the way to the ears.
Is that a good description of the differences between 3-wire and 4-wire TRS?
Your description of 4-wire cable is correct.
3-wire cable exists in 2 flavors, both bad:
type a) Shared GND + Left + Right go from TRS to a midpoint, where left and right split off. Ground splits into two. This is getting pretty rare and may only exist on some cheaper headphones.
type b) Shared GND + Left + Right go from TRS to one earcup. Inside the earcup, the shared ground connects to that transducer, and also to a ground wire coming through the headband from other transducer. e.g. Oppo PM3, NAD HP50 VISO, etc etc.
In both cases there's a portion of cable copper sharing ground return current for both transducers. That's the crosstalk mechanism. Current from one transducer will cause a voltage drop on this shared ground path, therefore causing the equal and opposite voltage to appear across the other transducer. The key to avoiding this is buying cables that have no portion of the cable carrying shared ground current. i.e. "4-wire".