Hi Kacey,
There's not long to go now until your trip - you must be so excited!
The good news is that yes, you can indeed add another day to your 3-Day Ticket with Park Hopper Option.
If it is within 14 days of a theme park ticket’s first use and the ticket has remaining admissions, you are able to upgrade the ticket. (If it is more than 14 days after first use or if no admissions remain, then a ticket may not be modified or upgraded.)
Upgrades may include:-
Upgrading a 1-Day Value Ticket to a Regular or Peak Ticket
Adding days
Adding the Park Hopper Option
Adding the Water Park Fun & More Option
Ticket upgrades must be made in person at the following locations at the Walt Disney World Resort:-
Theme park ticket windows
Guest Relations locations
Lobby Concierge at Disney Resort hotels
Wishing you a magical trip filled with memories which will last a lifetime!
Emma