This will Hide every third row starting with row 3 change the "For
i=3" to whatever row you want it to start at

Sub HideThirdRow()
fnl = Cells(65536, 1).End(xlUp).Row
For i = 3 To fnl Step 3
Cells(i, i).EntireRow.Hidden = True
Next i

End Sub

This will un-Hide the rows

Sub UnHideRows()
fnl = Cells(65536, 1).End(xlUp).Row
For i = 1 To fnl Step 2
Cells.EntireRow.Hidden = False
Next i

End Sub