This code doesn't loop from one worksheet to another.
The idea is to hide all rows which don't contain "X" in column A for all worksheets without three specified worksheets.
Option Explicit
Sub test()
Dim ws As Worksheet
Dim C As Range
For Each ws In ThisWorkbook.Worksheets
If ws.Name <> "Übersicht" Or ws.Name <> "Definitionen" Or ws.Name <> "Abkürzungen" Then
For Each C In Range("A6:A2000").Cells
If C.Value = "" Then
C.EntireRow.Hidden = True
Else
C.EntireRow.Hidden = False
End If
Next C
Else
End If
Next ws
End Sub