With the 3EM you can measure the 3 phases (3x400V). The power that is stored in the minute interval corresponds to the effective power of each phase. If you add all 3 together, you have the total power consumption. When installing, it is important that you clip the current clamps all in the "same" direction over the lines, otherwise phases can be counted as "feed-in" although it is a consumption.

Can I also use it to measure the consumption of individual circuits (behind the circuit breaker)?

Well, by 400V you probably just mean a 3-phase connection, right? If so, you can measure it with the 3EM by measuring each phase and then adding the wattage of the 3 phases.

Thank you;)! Max load 230v, too bad. But it should be suitable for normal house 230v circuits. Do you have your own experience?