$data = '347201247'
$newdata = ""
While 1
    If StringLen($data) <= 3 Then
        $newdata &= '+' & $data
        ExitLoop
    EndIf
    $newdata &= "+" & StringLeft($data, 3)
    $data = StringMid($data, 4, -1)
WEnd
MsgBox(0, '', $newdata)