There are three questions here. As you are in Germany, I assume you have a 4-wire electricity supply - three phases and neutral.
As the ‘Learn’ page about 3-phase supplies says, to measure 3-phase power you need one less wattmeter than the number of wires, so in your case, 3. For “wattmeter” you can use 3 × emonTx, or one emonTx using 3 current inputs. If you use 3 × emonTx, you will measure voltage and current of one phase in each emonTx, then you add the 3 power values to give the total. This is the most costly and the most accurate method.
If you have one emonTx, it has only one voltage input, so you must assume that the remaining two phases have the same voltage, otherwise there is an error. But it will measure the 3 phase currents accurately, and give a good estimate of the total real power.
The voltage you measure is totally dependent on the transformer (a.c. adapter) that you use. If you have a 380 V transformer connected line - line, then you can use that. You must not use our Euro a.c. adapter at 380 V.
The voltage rating for the YHDC SCT-013-000 refers to the insulation between the main cable and the secondary winding that is connected to the low voltage electronics. It is a safety standard. The value on the official YHDC data sheet is the transformer has been tested to 1 kV for 1 minute. You must not use this c.t. on a bare cable or busbar, therefore you will also have the insulation on the cable that is the primary winding.
(Seeedstudio has published wrong data in the past, so use their information with caution.)