BMW actually has two separate patents on these W3 designs, and both of them have the goal of cramming three cylinders into the space of a traditional V-twin. The first splays the cylinder out into a fan shape with pushrods operating the valves. The description submitted to the World Intellectual Property Organization describes the solution as, "The embodiment according to the invention represents an as of yet unknown type of W-3 reciprocating piston internal combustion engine having cylinder angles which can be largely freely sized."
The other solution is more of a modification to the traditional V-twin. Two of the cylinders share a crankpin, but the third has its own and is positioned inside the angle of the V. It's a very odd-looking engine.
With BMW's assertion that these designs are meant to fit in place of a traditional V-twin, Jalopnik speculates that they could be for a future cruiser from the company, and that would make sense. While brand offers a line of sport tourers with the K1600 line, it doesn't have something more classic to take on the likes of the Ducati Diavel. Creating such a weird engine would probably grab early interest from riders.