Since you probably won't be able to get a discount on a two-day ticket, your best bet would be just to upgrade the 1-day ticket into a 3-day ticket. Unless that ticket is specifically a Comp ticket, which has no value, you can upgrade it at Guest Relations and pay the difference in the price between the 1-day and the 3-day tickets.
If you bought the ticket before the recent price increase, make sure to use the ticket to enter a park before upgrading. That'll lock-in the current gate price value of the 1-day ticket.
-Rob