I remember seeing a 3wheel in-line cargo bicyle a looooooong time ago, but the third wheel was just a front fork in a headstock trailing the drive wheel.
check these out:
http://www.youtube.com/watch?v=W7DC4aJEBiA
http://www.cyclorama.net/viewArticle.php?id=354
http://www.youtube.com/watch?v=CJFw2La-m-o

Somehow you gotta be able to turn.
As a proof of concept go get 2 pcs of perforated angle stock(HomeDepot, Lowes, L HDW), long enough to go from trailing 3rd wheel axle bolts to rear axle bolts or even chainstays of drive bike, clamp it together with U-bolts and see if you can ride it. You might want to borrow some bikes and another front wheel from someone who wont miss them, JIC.