That is the name you give to identify the Node
No - you don’t need that Node Definition. The data comes in “Key:Value” format, so there is no need to decode the value and attach a name to it, which is what this part of emonHub does.
You have done so, for the first one: it is connected to port USB0 and you have named it “emonTx4” in the Interfacer. You need two more USB ports and two more instances of the Interfacer for the other two emonTx V3s. You might want to change the first to “emonTx1” and name the others “emonTx2” and “emonTx3”. Or maybe emonTx0, emonTx1 & emonTx2 to match the USB ports?
So that the emonTx’s don’t get swapped around if the port names are allocated to a different physical port when the emonPi reboots, look at the part in this thread where “udev rules” are discussed. (Note: where they use “3No.”, they mean a quantity of three.)