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