Hi, Julie! You have a few options for purchasing tickets. You can purchase them online
HERE or at the parks for the same price (shipping not included if you want them mailed to you; free will call pickup available.)
Another option is to purchase through www.undercovertourist.com. This site sells tickets at a slight discount, but where the savings comes in is there is no tax or shipping charges. While ticket prices are always subject to change, a 3 adult base ticket is about $233 and 3 day child (ages 3 - 9) base ticket is $199. If you plan to visit more than one park in a day, you will want to add on the Park Hopper option to each ticket which is about $51 per ticket. With just 3 days, you might just want to pick one park a day and stick to it. It will save you time and money.
A third option is to go to your local AAA office where discounted tickets are sold. This discount is available only at the AAA office, AAA discounts are not given at the ticket windows in WDW.
Whichever way you decide to get your tickets, I suggest doing ahead of time to save you time. Once you reach WDW, you can get to the magic right away without waiting in line to purchase tickets. One last note: if you plan to book a resort package including the Disney Dining Plan, you will need to purchase your tickets as part of a package through Disney.
I am so happy to answer your question, stop by anytime!