With only three wheels on the ground the rear stiffness is controlling the roll angle, which is determining how high the inside front is off the ground (ignoring longitudinal load transfer). The force exerted by the road on the outside front tire does create a moment about the mass center (which we will assume is on the plane of lateral symmetry of the vehicle), but that moment does not change beyond small sine/cosine effects due to roll angle. The reason is that the weight on that tire does not change (and the track is assumed to be constant). Ignoring mass concentrations,aerodynamic forces, and maneuvers resulting in z axis acceleration the front to rear weight distribution must remain the same for the car to be in pitch equilibrium. If you have only 1 front wheel then there is only one load that can be on that corner. If that load is different then you are accelerating in pitch.
What happens is that as you continue to increase lateral acceleration on three wheels the weight transfers off the inside rear to the outside rear, causing the outside rear to compress and lift the inside front even further due to a combination of roll and pitch. Then you end up on 2 wheels and the dynamics of the problem get a lot more complicated.
