Increment months in next available column, but multiple rows - excel

Question

I have VBA code which increments dates in the active cell to the next available column.
Dim lngLastCol As Long, lngRow As Long
lngRow = ActiveCell.Row
lngLastCol = Cells(lngRow, Columns.Count).End(xlToLeft).Column
If IsDate(Cells(lngRow, lngLastCol)) Then
With Cells(lngRow, lngLastCol + 1)
.Value = DateAdd("M", 1, CDate(Cells(lngRow, lngLastCol)))
.NumberFormat = Cells(lngRow, lngLastCol).NumberFormat
End With
End If
Instead of incrementing the month (and year) on the active row I am currently clicked on, I want to update the months in certain fixed rows i.e. Row 3, 17 and 42 (all in the same column).

Answer 1

Another approach, loop one column to the last row (in this case 250). In the second If formula you set which rows to add new columns to. So if this statement is true Cells(i, 2).Row = 3 (current row we loop is 3) then add a new column.
Therefore we replace the active row with a loop:
lngRow = ActiveCell.Row -> lngRow = Cells(i, 2).Row
The For i loop will from row 3 to row 250.
Sub ColumnsAdd()
Dim lngLastCol As Long, lngRow As Long, i As Long
For i = 3 To 250 'Loop from row 3 to 250
If Cells(i, 2).Row = 3 Or Cells(i, 2).Row = 17 Or Cells(i, 2).Row = 42 Then 'If any of the rows is 3, 17 or 42 then go and add new column
lngRow = Cells(i, 2).Row
lngLastCol = Cells(lngRow, Columns.Count).End(xlToLeft).Column
If IsDate(Cells(lngRow, lngLastCol)) Then
With Cells(lngRow, lngLastCol + 1)
.Value = DateAdd("M", 1, CDate(Cells(lngRow, lngLastCol)))
.NumberFormat = Cells(lngRow, lngLastCol).NumberFormat
End With
End If
End If
Next i
End Sub

Answer 2

I dont understand exactly what to you want but you can use the below code and if you want more adjustment let me know.
Option Explicit
Sub test()
Dim lngLastCol As Long, lngRow As Long
lngRow = ActiveCell.Row
lngLastCol = Cells(lngRow, Columns.Count).End(xlToLeft).Column
If IsDate(Cells(lngRow, lngLastCol)) Then
With Union(Cells(3, lngLastCol + 1), Cells(17, lngLastCol + 1), Cells(42, lngLastCol + 1))
.Value = DateAdd("M", 1, CDate(Cells(lngRow, lngLastCol)))
.NumberFormat = Cells(lngRow, lngLastCol).NumberFormat
End With
End If
End Sub