Hi there Pablo,
Welcome to planDisney and thanks so much for your question! I'm thrilled to hear you're planning a trip to the Disneyland Resort early next year!
When I visit the parks with my family, we love adding the
Park Hopper option - it gives us a lot of flexibility with our days to jump between parks for different attractions, meal options, and entertainment. When you purchase this option while buying your tickets, it does apply to all days of that ticket.
It sounds like you all are putting together the itinerary for your trip and want to stick to one park per day for the first two days of your trip, but wanted to see if you could add the Park Hopper option for your final day.
To do this, you would need to go to one of the ticket counters outside the parks at the beginning of your final day to add on the Park Hopper option for that day. The caveat here is that it will depend on whether or not there are available
theme park reservations for that day. If there are no theme park reservations available for that day, then you would unfortunately not be able to add on the Park Hopper option for the day.
Having said that, my recommendation would be to first assess how important the Park Hopper option is to you and your family. If it's very important and your budget allows it, then my suggestion would be to purchase
3-day tickets with the Park Hopper option. This would give your family the flexibility to cross between the parks after 11:00 AM for each day of your trip and is generally my preferred way to experience the parks.
If you are okay with the potential of not being able to add on that Park Hopper option for the final day of your trip if there are no available theme park reservations, then I would do exactly as you mentioned in your question! Just purchase standard 3-day tickets then on your final day, a member of your party can go to a ticket booth to check if you all would be able to add on that Park Hopper option.
Whether you visit one park per day or park hop, I'm certain you all are going to have a fantastic time visiting the Happiest Place on Earth! Thank you again for your question, Pablo, and please feel free to come back and see us here at planDisney if you think of any follow-ups as you continue to plan.
Have a Magical Day,
Greg