If you're not using it, this is the website for the museums:
https://www.uffizi.it/en
Click "tickets" on the upper right of the page and the next page comes up; it explains the ticketing options:
https://www.uffizi.it/en/tickets
So as you can see from that page, a single adult ticket for the Uffizi is 20€ +4€ reservation fee = 24€
Reduced tickets only apply to European Union citizens aged 18 -26.
Children under 18 are "free" with 4€ reservation fee, as TravelisFun said above,
A single adult ticket for 3 consecutive days for Uffizi, Pitti Palace and Boboli Gardens is 38€+4€ reservation fee.
Click "buy online" and you'll be taken to the b-ticket website for making your purchase. If it doesn't come up in English, there is a tab for that at the bottom of the page.
If you want the 3-day combo pass, just choose the Uffizi as it'll be offered as an option along with tickets for just the museum itself. Choose your day and desired entry time for the Uffizi (mandatory) and number of "Intero/Full" tickets if you are adults wanting to visit JUST that museum, or" Intero Cumulativo 3 giorni" if you want the 3-day option that includes the Pitti and Boboli.
From what I understand, the 3 day does include the Uffizi, Pitti and
the Boboli gardens but you are choosing the time for the Uffizi when
you book it. Hopefully, someone will correct me if this is wrong.
Yes, you are only choosing an entry day/time for the Uffizi. The other two can be visited any time during the life of the 3-day ticket.