I think it's going to be hard to fit three kids in any trailer. If you look at the pictures of that Schwinn, it looks pretty crowded with just the 4 year old and what looks like about a 1 year old. How often does your 4 year old ride in a stroller? Have you considered using an Ergo or other carrier with your youngest during those times your 4 year old wants to hitch a ride so that you wouldn't need to sit three across?
Those both seem like good choices for two kids. If you can find one used, Trek used to make a "GoBug" trailer that's pretty large and I think it had a stroller conversion.
I've also heard good things about Wike trailers as far as space:
http://www.wicycle.com/child_trailers.php There's a family that just finished cycling across the U.S. and to South America using a Wike trailer for their 5 year old and 2 year old:
http://www.pedalpoweredfamily.com/20...child-trailer/