try this: 
 #include <GUIConstants.au3>
$Form1 = GUICreate("AForm1", 625, 445, 193, 115)
$Tab1 = GUICtrlCreateTab(80, 80, 393, 209)
$tabitem1 = GUICtrlCreateTabItem("Tab1")
$tabitem2 = GUICtrlCreateTabItem("Tab2")
$tabitem3 = GUICtrlCreateTabItem("Tab3")
GUICtrlCreateTabItem("")
$Button1 = GUICtrlCreateButton("Create Button to:", 112, 336, 137, 33, 0)
$combo1 = GUICtrlCreateCombo("", 250, 340, 137, 33, $CBS_DROPDOWNLIST)
GUICtrlSetData(-1, "Tab1|Tab2|Tab3", "Tab1")
GUISetState(@SW_SHOW)
While 1
	$nMsg = GUIGetMsg()
	Switch $nMsg
		Case $GUI_EVENT_CLOSE
			Exit
		case $Button1
			$var = guictrlread($combo1)
			if $var = "Tab1" Then
				GUISwitch($Form1,$tabitem1)
			ElseIf $var = "Tab2" Then
				GUISwitch($Form1,$tabitem2)
			Else
				GUISwitch($Form1,$tabitem3)
			EndIf
			GUICtrlCreateButton("OK!", 300, 200)
			GUICtrlCreateTabItem("")
	EndSwitch
WEnd