I've thought about this, and touched on it in the thread I linked to in post #1. I think that degree of difficulty of turning will be dependent on the ratio of distance between the 2 front wheels and the distance from the center point of the 2 front wheels to the rear wheel. Plus rolling resistance of the rear wheel, and air resistance along the sides, increasing with speed. I think that a traditional "aerodynamic" body style (long, slender, tapered toward the rear) would make this vehicle very hard to turn, especially at high speed. The easiest body style to turn I think would be a hemisphere, like an upside down bowl. I don't know how efficient that would be though. Definitely an area which deserves more thought.I think you are going to have problems if you don't at least have pivoting wheels. A rigidly mounted wheel wants to track straight ahead. Turning two wheels at different speeds on a common axis is not too much of a problem and the object can turn about the vertical axis that passes through the common axis pretty freely. But if you add another wheel that does not share that common axis, then it will fight you. This is true even in aircraft that use differential braking on the main gear to steer -- the nose/tail wheel still needs to pivot.
I guess it wouldn't hurt to familiarize myself with how a regular car works before reinventing the wheel.In a conventional four wheeled vehicled, the two front wheels not only need to pivot, but they need to pivot by different amounts. because you want the axes of all four wheels to pass through a common point at the center of the turn. The steering linkages, such as the Ackerman steering arm, are pretty amazing when you understand the subtleties of what they do.