I assume a threenager is a 3-year-old?
I've been close -- my daughter was 4 on her first visit. As
@SAV indicated, she acted the same way that she does at home. (In other words, if she was overtired, hot, uncomfortable or hungry, she got whiny and petulant. Otherwise, she was good.)
We kept the whiny moments to the bare minimum (and I mean, the bare minimum - she did amazingly well and we had no meltdowns whatsoever), by keeping fatigue, discomfort, heat and hunger at bay. We made sure that she got to bed around the same time as at home, every single night (except for the last night when we all stayed up for the MK fireworks). We kept up a daily schedule that included getting to a park at rope drop and touring for about 4 hours, stopping for a snack or for lunch when our tummies dictated, then heading back to the hotel for 3-4 hours of naps and swim time in order to stave off fatigue and bypass the worst heat of the day. (She stopped napping at home at age 2, but the entire family always naps daily at WDW, and enjoys it!) Then we'd spend a more leisurely evening in a park of our choice, with a reasonable bedtime. We also managed expectations so there would be no big disappointments: we educated her before we left about what attractions and dining fit into our plans and budget, and which items did not. We let her choose 3 attractions and 3 characters that were "must-do" items for her, so she got to feel included in the planning and decision-making. We also had lots of talks (and shared stories) about the fact that vacations always include hiccups -- a favorite attraction might be closed, a desired snack might have been taken off the menu, or a late bus might mean missing a parade -- and that we need to be able to shrug and say, "Oh, well," when those things happen. (She took this lesson to heart a little too well -- for the next few weeks, whenever she was caught in the act of doing something wrong, she'd chirp, "So now we just say, 'oh well,' Mommy, right?") She also know that, just as at home, if she was disobedient, there would be negative consequences, like going to bed early or losing the privilege of selecting a small souvenir to take home.
It worked well, for us. Our daughter still acted like a 4-year-old, but a pretty well-behaved 4-year-old at that.