Hi Mia and welcome to planDisney.
It looks as though you're only a few weeks away from your visit to 
Walt Disney World - it doesn't get much more exciting than that!
When purchasing Park Tickets from the UK version of the Walt Disney World website, it is only currently possible to purchase our UK & Ireland Exclusive 7-Day or, 14-Day, Disney Magic Tickets. These tickets offer incredible value for money allowing you to come and go between the Disney Theme Parks and Waters Parks, as and when you like, on any 7, or, 14 days, (depending on which ticket you buy) within the usage window of your ticket.
So, for example, if you were to purchase a 
7-Day Disney Magic Ticket with a start date of 20 November 2025, this ticket would be valid for use on any 7 days from 20 November - 4 December 2025. 
These tickets also include lots of added extras including the ability to park hop, a free game of miniature golf at Disney's Winter Summerland or Disney's Fantasia Gardens (includes one round per day, access prior to 4pm) and my personal favourite, Memory Maker. I have literally thousands of lovely photos of my family at Walt Disney World from over the years - most of which were included on 
Memory Maker.
As an alternative, it is indeed possible to purchase a 3-day Ticket with Park Hopper Option via the US version of the Walt Disney World website or, My Disney Experience Mobile App, once you have arrived in the US.
Although prices are always subject to change, to give you an idea, the current cost of a 3-day Ticket with Park Hopper Option, valid on any 3 days from 20 - 24 November 2025 is $664 (inc tax) per Guest Ages 10+ and $648.98 (inc tax) per Guest Ages 3 - 9. 
This ticket would enable you to visit all 4 Theme Parks (Magic Kingdom Park, EPCOT, Disney's Hollywood Studios and Disney's Animal Kingdom) and the Park Hopper option allows you to visit more than one Theme Park per day (subject to the park’s capacity limitations).
I just did a little currency conversion and you might be surprised to know that as of right now, it would only cost approximately £38 extra, per Guest Ages 10+ and £20 extra, per Guest Ages 3 - 9, to buy the 7-Day Disney Magic Ticket as opposed to the 3-Day Ticket with Park Hopper Option. 
I hope that this information has been of use, Mia. Please feel free to re-visit us, here at planDisney, if we can be of any further help with the planning of your trip.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma