In future it would be better to post executable code as you probably would get more help.
With that being said I assume you were after something like below?
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
#include <GuiListView.au3>
#include <ListViewConstants.au3>
Local $iSelection = "", $aSelection = ""
Local $hGui = GUICreate("Benutzersteuerung", 612, 460, 189, 132)
GUICtrlCreateLabel("Vorname", 12, 8, 70, 17)
Local $idFirstName = GUICtrlCreateInput("", 90, 4, 146, 21)
GUICtrlCreateLabel("Nachname", 12, 32, 70, 17)
Local $idLastName = GUICtrlCreateInput("", 90, 28, 146, 21)
GUICtrlCreateLabel("Badgenummer", 12, 56, 70, 17)
Local $idBadgeNumber = GUICtrlCreateInput("", 90, 52, 146, 21)
Local $idAddButton = GUICtrlCreateButton("ADD", 266, 8, 60, 25)
Local $idDelButton = GUICtrlCreateButton("DEL", 330, 8, 60, 25)
Local $idModButton = GUICtrlCreateButton("MOD", 266, 40, 60, 25)
Local $idSearchButton = GUICtrlCreateButton("SEARCH", 330, 40, 60, 25)
Local $idListView = GUICtrlCreateListView("Badgenummer|Vorname|Nachname", 0, 75, 609, 383, BitOR($LVS_EDITLABELS, $LVS_SINGLESEL))
_GUICtrlListView_SetColumnWidth($idListView, 0, 150)
GUISetState()
While 1
$nMsg = GUIGetMsg()
Switch $nMsg
Case $GUI_EVENT_CLOSE
Exit
Case $idAddButton
$sGuiInputData = ""
$sGuiInputData &= GUICtrlRead($idFirstName) & "|"
$sGuiInputData &= GUICtrlRead($idLastName) & "|"
$sGuiInputData &= GUICtrlRead($idBadgeNumber)
GUICtrlCreateListViewItem($sGuiInputData, $idListView)
GUICtrlSetData($idFirstName, "")
GUICtrlSetData($idLastName, "")
GUICtrlSetData($idBadgeNumber, "")
Case $idDelButton
_GUICtrlListView_DeleteItemsSelected($idListView)
Case $idModButton
Switch GUICtrlRead($idModButton)
Case "Mod"
$iSelection = _GUICtrlListView_GetSelectionMark($idListView)
If $iSelection = -1 Then ContinueLoop
$aSelection = _GUICtrlListView_GetItemTextArray($idListView, $iSelection)
If $aSelection[0] = 0 Then ContinueLoop
GUICtrlSetState($idAddButton, $GUI_DISABLE)
GUICtrlSetState($idDelButton, $GUI_DISABLE)
GUICtrlSetState($idSearchButton, $GUI_DISABLE)
GUICtrlSetData($idModButton, "Update")
GUICtrlSetData($idFirstName, $aSelection[1])
GUICtrlSetData($idLastName, $aSelection[2])
GUICtrlSetData($idBadgeNumber, $aSelection[3])
Case "Update"
_GUICtrlListView_SetItemText($idListView, $iSelection, GUICtrlRead($idFirstName), 0)
_GUICtrlListView_SetItemText($idListView, $iSelection, GUICtrlRead($idLastName), 1)
_GUICtrlListView_SetItemText($idListView, $iSelection, GUICtrlRead($idBadgeNumber), 2)
GUICtrlSetData($idModButton, "Mod")
GUICtrlSetState($idAddButton, $GUI_ENABLE)
GUICtrlSetState($idDelButton, $GUI_ENABLE)
GUICtrlSetState($idSearchButton, $GUI_ENABLE)
$iSelection = ""
$aSelection = ""
EndSwitch
EndSwitch
WEnd