The requirement for three sampling tubes per parameter per room is based on guidance in Annex D of ISO 16000-2, covering the dependence of the confidence interval for the determination of formaldehyde in indoor air.
Where deemed appropriate by the accredited organisation performing the sampling work, it is acceptable for one sampling tube to be used in each room for each of the formaldehyde and TVOC measurements (i.e. two per room if both parameters are being measured).