Yes the 3 year old is supposed to have a ticket, is Disney going to question you? they might. Are they going to require proof, only if the child looks way over 3.
I just wanted to add for all the people out there who think they can look at a kid and tell their age, you can't. Some kids are big for their age, and some are small for their age. Example. My cousin has always been very tall for her age, and when I say tall, not the 95th percentile, she was off the charts. At 2, she wore a size 6X, (because she needed the extra legnth), and was over a head taller than all the other kids in her daycare. I remember picking her up from school when she was in kindergarden. They would be walking in line, and they next kid would be at her shoulders. Once I took her somewhere, and 13 and over was a certain price, and under 13 was a certain price. She was 8. The lady selling the tickets argued with me, and my cousin cried the whole time because she was so embarrassed. Afterthat, I just paid more to spare my cousin tears.
I am the opposite, my mom had to convince people that I was really 3! DD is the same way.
Point, you can judge someone on how big they are. Some 2 year olds are the size of 5 year olds, and in that case, bring the kids b/c.