Expression to change an attribute each X number of rows

Question

I'm using QGIS and I need an expression to change an attribute each X number of rows. For instance I have the next attribute data:

I want to create an attribute (C) that assigns an ordered value each 3 rows. With the next result:

Answer 1

Assuming your rows are already sorted, you just have to create a new integer field with the expression: ($rownum + 1) / 3