If you are employed in three-shift periodical work, your working hours are reviewed per adjustment period. If your working hours are, on average, at least 18 hours per each week of the adjustment period, each of these weeks will be counted towards your employment condition.
Example
The Working Hours Act defines the sectors in which periodical work can be applied. Merely the fact that your employment contract states that your work is periodical does not qualify it as periodical work.