need help on data highlighting

Dear All,
I am a newbie in writing VBA and has absolutely no clue to how to write one (apart from relying on tutorials). I have written the following and got stuck

Sub test()

Dim rng As Range
Dim x As Long
For x = 2 To 100 Step 6
If rng Is Nothing Then
Set rng = Range(Cells(x, 1), Cells(x, 18))
Else: Set rng = Union(rng, Range(Cells(x, 1), Cells(x, 18)))
End If
Next x
rng.Select
    With Selection.Interior
        .Pattern = xlSolid
        .PatternColorIndex = xlAutomatic
        .ThemeColor = xlThemeColorDark1
        .TintAndShade = -0.149998474074526
        .PatternTintAndShade = 0
    End With
End Sub

My spreadsheet consist of 3 rows of numbers for every sample, and I want to highlight with grey every three rows at a time to tell one apart from another. With the current strings I can only highlight one row, anyone knows how to expand the selection?

Hi Wilson181085, and welcome

without even trying to execute the code it does appear that the "rng.select to end with should be before the Next x.

Suggest you shift the Next x line to the second last and try again.

You should also know that it will assist you getting a solution to any problem you are stuck on if you include a sample workbook showing the starting point and the expected results. It doesnt have to be a big sample, just enough to make your intentions crystal clear to the volunteers who assist on the forum

I'm not sure what you are doing, you say every 3 rows, but your loop is stepping every 6.

set rng = Range(Cells(x, 1), Cells(x + 2 , 18))

will select rows 2, 3 & 4 for the first iteration, then 8,9 & 10 for the 2^nd, etc