From the time the kids were 3, 5, 6, and 7, they decided how they would spend their money at WDW. When they were younger, I held their envelopes with the cash (each one marked with their name). When they wanted something, I assisted with the transaction. When oldest DD was 8, she spent all her money the first day on jewelry. Her choice, and I told her she was using all her money early on. She said that's what she wanted to do, so okay. On the next to last day, she saw something she really wanted and asked to buy it. I asked how much money she had left. She thought about it and put the item back, saying "Maybe next year." Excellent money lesson taught right then and there. As they've gotten older, they carry their money with them. If they lose any of it, it's their problem not mine. It's been interesting to watch my two DDs who are low on funds pull their money to buy a toy to share. They've learned how to negotiate, which continued when they got home on who would play with the toy when.
I have always let them buy whatever they want with their own money. Hard to keep my mouth shut sometimes, but it's satisfying to see them learn what is "junk" and what is a "better buy." Again, a good money lesson learned at a low price point and safe learning environment.
As for how much money, it has always varied due to when birthdays fall and what has been earned doing chores.
I would hold the 3 year old's money, but let her pick what she wants...within reason!
Have a great trip!