Why stall on 2D faster than 3D?
The 3D-wing allows for air to flow over the wingtip into the low pressure zone over the wing, thus reducing the overall pressure differential and reducing overall lift.

This explains the stall at higher AoA because the additional airflow over the wing surface "energises" the air. Flow seperation happens when the kinetic energy is equal to the pressure differential that the airstream flows into, so if you can increase the kinetic energy, you can decrease the tendency of the airstream to separate.