I would never recommend that someone cheat the system like this with an older child (5+) that planned to ride most of the rides . . . but you could always say to the attendant checking the passes that your 3 year old isn't riding but rather staying in the child swap area. Then, once in line, just let them ride if they are tall enough.
Honestly, I doubt anyone would even ask for your 3 year olds express pass if everyone else had them. If she was a bit older, then you'd probably need to get her a pass. I'd roll the dice and save the money if I were you. Not worth it for 1 or 2 rides.