Royal Greetings, Alexis!
Welcome to planDisney! I'm so glad you're here! How exciting that you're planning a summer visit next year to celebrate your daughter's third birthday...HOT DOG! That's going to be one 
Tigger-iffic celebration! Just thinking about it brings back memories of when my (not-so) little ones were that age. We've celebrated so many special milestones at the Most Magical Place on Earth, and those moments have truly become some of my most treasured memories. I'm so happy you'll soon be creating those same kinds of magical moments with your own family! 
Once your Mouseketeer turns three years old, they're officially old enough to need a 
theme park ticket at Walt Disney World, even if that birthday happens right in the middle of your trip. However, there's one magical exception for Guests who book a 
vacation package. In that case, your child's age is determined at check-in and stays that way for the entire vacation. So, if your little one checks in as a two-year-old but celebrates their third birthday during your stay, they'll be considered two for your whole visit. This means they won't need a theme park ticket...they'll be treated as a special Guest of Mickey Mouse!
Now, if you're purchasing theme park tickets separately from your resort stay, the rules work a bit differently. Your daughter would need a theme park ticket once she turns three years old.
And speaking of turning three...what a magical age to visit 
Bibbidi Bobbidi Boutique! This royal experience is designed for royal princes and princesses ages 3-12, so once your daughter officially celebrates that big third birthday, she can enjoy her very own fairytale makeover! If you're staying on a vacation package, you can simply book the reservation under your name and include her as one of the participating children.
Here's a little tip: After your daughter's magical transformation at Bibbidi Bobbidi Boutique, make your way over to the Royal Portrait Studio at 
Sir Mickey's. There, a 
Disney PhotoPass photographer can capture some photos of your little princess! 
Alexis, I hope this helps as you continue to plan your trip! Feel free to stop by planDisney if you have any other questions throughout your planning! We're always happy to help!
Magical Wishes,
Kami