We went in 1999 when the boys were 3 & 5 and they absolutely loved it. We spent a full day there, and if the park was not closing, we could have spent longer.
Went again last year when the boys were 7 & 9, and they enjoyed themselves just as much.
There is loads for a 3 year old to do, but a few of them involve getting wet, so I would advise a change of clothes for your child.