Hi, Kathryn!
Happy birthday to your little princess!
Guests of Walt Disney World who are under three years of age do not need to purchase a ticket for entry into a WDW park. As such, when your little one turns three, it's easy to head to a ticket window at one of the WDW Parks or Downtown Disney in order to purchase her a park ticket, should you wish to enter a park at that time. No need to do so before she turns three. :-)
For some great tips on traveling with
Little Ones at Disney Theme Parks, take a look at this post on Walt Disney World's website. It's filled with helpful information, links and so much more; you'll be so glad you read it!
Also, be sure to pick up a complimentary Birthday Button upon check-in at your resort or at Guest Relations in one of the parks. Your birthday girl will hear "happy birthday!" from Cast Members and Guests alike; what a wonderful way to celebrate...all day long!
Send more questions as you think of them, too, Kathryn; we love to help! And be sure to wish your little a very happy birthday from all of us Moms and Dads on the Disney Parks Moms Panel!