'Трябва да ги сложи в ComboBox3 taka:
ComboBox3.addItem "sk_materials"
ComboBox3.addItem "sk_stocks"
......
.....


Private Function CheckIfSheetExist(SheetName As String)
    Dim ws As Worksheet ', sheetName As String
    For Each ws In ThisWorkbook.Worksheets
        If ws.Name = SheetName Then
            CheckIfSheetExist = True
            Exit For
        Else
            CheckIfSheetExist = False
        End If
    Next ws
End Function