If, by chance, you actually have 2 separate USB 3.0 front panel connectors and only one internal USB 3.0 header, you can use a converter like the one linked below to connect to your third USB 2.0 connector (it will only run at USB 2.0 speeds as well).
http://www.newegg.com/Product/Product.aspx?Item=9SIA2970X40954