To fill cells down with a fixed step value (e.g., 3), please follow the steps below for details:
Step 1: Select the range with the data on the top;
Step 2: Click the "Home" tab from the ribbon;
Step 3: Click "Fill" in the "Editing" section, and "Series" from the drop-down list;
Step 4: In the "Series" window, click "Columns" and "Linear", then type a step number (e.g., 3) in the box; you can also make a stop number in the "Stop value" box.
Step 5: The data range is filled with data and step values.