Antonio,
Thanks for asking your question to planDisney!
The way that the park hopper option works is that you purchase it for the duration of your ticket package. There is no way to purchase the option for just 1 of the 3 days.
The good news for you is that the cost difference in purchasing a 3-day park hopper option as opposed to a 1-day park hopper option is just $10 US, at the time of this answer. So, you are not talking about a large difference in price, and you get some amazing flexibility for those two other park days.
Now prices can change over time, but even if they do, I expect you will find that the added cost of the park hopper is well worth it. Since you are traveling in from such a long distance, making sure you get to visit all four parks is a great idea!
BONUS TIP #1On days where you are at either
EPCOT or
Disney's Hollywood Studios, park hopping between the two using the
Disney Skyliner is a snap! Like with the
monorail connecting EPCOT and
The Magic Kingdom Park, the ride is not just convenient, but an attraction unto itself.
BONUS TIP #2
Regardless of where you spend your days, consider evening visits to EPCOT for some of the great dining choices. Even just walking around the World Showcase pavilions and grabbing some small bites is a great way to both explore some new flavors and enjoy the park atmosphere.
Have a Magical Vacation!Doug