The ET^3 is an Ethernet cable meant for audio. It can be used for i2s connections which use standard Ethernet wiring, or for LAN connections like from a router or NAS to any (streaming) device.
The ET^3 is currently the only ^3 series cable from Phasure; the ET^3 contains 4 fully configurable screens / shields, each insulated from the other. The ET^3, like the ET^2 is extra special because it provides "output ground" wires to connect to the respective devices, with that forming additional shielding. Do notice that this is not Ethernet-common at all and that you can use it as an option - both sides, one side only, or none at all. Crocodile wires are provided to quickly test the ground wire(s) and if a suitably sounding setup has been found, the eyes to the end of the ground wires can be screwed more permanently to any suitable means for it.
Take notice : While the grounding wires do not provide ground to the Ethernet wires anywhere (be that the i2s or the LAN application or other), they will provide a ground connection between the two connected devises when both ends are connected to the respective device chassis indeed. That is, assumed the chassis is what the ground wires are connected to. It is therefore important to know that in normal applications it is not so that chassis of the two devices in order are connected and it merely is so that a general Ethernet application will galvanically isolate (this will not be the case, generally, for an i2s application). It is therefore recommended that each of the end of the extra grounding wires can be connected to its respective device all right, but that it better not be both at the same time. On again the other hand, nothing will break or go wrong if you do connect both ends.
Of course you understood that while the extra grounding wires could physically be connected to their devices, it is the B(lack) wire of the ^3 (and ^2) topology which determines whether the ground wire really is connected through and if so, by means of which of the screens (W(hite) = Inner, Y(ellow) = Second or R(ed) = Third, G(reen) = Outer). So yes, compared to the ET^2 version with one shield less, a new connector has been developed as well, so that really all the connection combinations are available to you; Do notice the position (over pin 4-5) of the jumper in the pictures shown, which connect the two separated parts of this new connector, so that all 5 wires can connect to each other indeed.
Important : The extra ground wires are shielded wires in itself, the shield not being connected to the "eye" at the ends. If you'd use the crocodile wires to test things or extend the ground wire with it, it is good to understand that the crocodile wires are not shielded.
Specifications (extended STP7) - Shielded Twisted pair (foil shield per pair (4x), 26AWG) - General Foil + Braid, 100% coverage - 3 extra screens (shields), each with 100% coverage, all shields insulated from each other - Plastic connectors to avoid parasitic capacitance when no device-device shield connection - Shielded ground wires at both ends for optional device-device connection via configurable cable shields - 3600 (?) configuration possibilities - Cable diameter ~12mm - Bending characteristic : 6cm radius (14cm outside diameter) - Obtainable to 10m on request; longer on special request.