I'm assuming these were a gift to you? So long as they are not some type of special promotional ticket that says you can't upgrade it, you will be able to upgrade them to 3-day park-hoppers. You will have to pay the difference between the gate cost of a 3-day park-hopper and the price that was paid for the tickets that you received (not necessarily the value of a 2-day park-hopper at the gate). Since they were a gift, I'm not sure if you know that value or not?