http://www.youtube.com/watch?v=me44yhEk2Bo

I didn't know that there's only one pilot on board on the Trislander! I've always been fascinated by the design of the Trislander; it looks like a mini DC-10! I wonder why they decided to go with 3 piston engines instead of 3 turboprop engines.

Speaking of piston engines, are they basically the same as the older radial engines on, say, the DC-3?

Not quite. The Trislander uses the Lycoming IO-540, rated at 290 HP. The DC-3 used(s) primarily the Pratt & Whitney Twin Wasp S1C3G radial, rated at 1,200 HP. Two completely different categories of plane. One with a gross weight of 10,000 lbs, with the other roughly at 26,000.

Why Britten-Norman chose to go with a horizontally-opposed piston engine instead of a turbine is likely due to economics.

Interesting. I thought that props were more economical then piston engines, though?

I think you mean "turbines" being more economical. That's true in general for certain mission profiles. But for what the Trislander was designed to do, the Lycoming has served it well while keeping costs low.