In the vehicle registration certificate ('vognkort'), it is stated that the permissible payload ('tillatt nyttelast') including passengers is 570 kg. The driver is not considered part of the payload. Each passenger is assumed to weigh 75 kg.
You find the answer to this question by deducting the weight of 3 passengers (3 x 75 kg) from the maximum permissible payload:
570 kg - 3 x 75 kg = 345 kg.