I had a great double stroller, but I still hated it. They are so huge (hard to put in the trunk, hard to navigate in stores).
I have a better suggestion than a double stroller. Since my daughter turned 3, we ditched the double stroller and used a regular stroller with a Buggy Board. It is a "board" on wheels that attaches to the back of any stroller (you can even switch between your regular stroller and an umbrella stroller). Your 3 year old can either stand on it or sit sideways on it (while holding the stroller for balance). It's great because 3 year olds don't really want to sit in a stroller for long periods. They usually would rather walk. With the Buggy Board, though, they have the option of riding when they get tired or when you need better control of them. My daughter absolutely loves hers, and everytime we are out, numerous people stop us and say, "Wow. That's great. Where did you get it? I wish I'd had one for my kids."
The brand is Lascal, and you can get it at Buy Buy Baby on Hall Road in Utica or you can order one online. They run about $90 (less than a double stroller!).
If you do get one, make sure you get the right one. There are two models depending on the set-up of the bars on the back of your stroller. I just took my stroller in to the store and had an employee attach it for me there to make sure it was right.