I guess the big question is how big is your 3 year old. Could you just use a wagon when you go to the zoo and avoid a stroller? Are you really going to be in lots of situations where you need the double stroller?
I had both of these. First I had the front & back double stroller which worked well when there was somebody sleeping. Then when they got older, there was lots of hair pulling, so I got the side by side.
The side by side was great in wide open spaces like the zoo, but truly a pain in stores and other small confined spaces. There did seem to be more legroom for older kids on it.
I never had the sit and stand stroller, but some of my friends swore by it. They didn't use it with an infant, they had a few toddlers.
M.