I think ages 3 and 5 will enjoy the park. I think age 5 is a really great age. If your 3 year old takes naps typically then it might be a long day for them but you can take a stroller or I am pretty sure you can rent one of the park strollers. My biggest issue next to the price is the long line ups which is enough for me not to take my kids back. They were 8 and 7 when we went and even they were bored with the waiting around. It's a lot of money for 1 day in the park and 2 nights accommodation, but you are paying the premium because it's a 1 day ticket. I personally liked Universal a lot more.

On a side note, a terrific place to take children under 7 is Sesame Place in PA. It is lots cheaper than Disney but an amazing place with no shortage of things to do. The whole park is made for children from the age of walking to age 7. Beyond age 7 it is still great but those older will not be able to go on everything...I just love that the little ones have been completely catered for. The water park at Sesame Place has been built with lots of thought too. Even 2 year olds can go on the rides as the slides for example have been designed in a way that upon exit, the child will not go under the water, and other slides have a double tube so parents can accompany the younger ones down it for reassurance. They have a parade there just like Disney and it's really big and interactive and there is meet and greet lunch and dinner options and photo ops all over the park. The characters are always walking around the park interacting with the children too...I know your current trip to Disney is already on the cards but for future reference if you want a great family experience, Sesame Place is amazing!!