The 2D model is incorrect for a 3 point bending model because your displacement has selected the entire top edge and not a vertex at the center.  You can create a vertex at the center in DesignModeler by splitting the edge.

The 3D model has created a narrow face to push on at the center. It would be better to put a remote point on that face and push down on the remote point, which will distribute the force to the face.

The 3D model also is holding two faces on the bottom with 0,0,0 displacement. That does not represent a 3-point bending test that is typically on rollers. Please review this discussion and come back with any questions.

The material you call concrete has no failure mechanism in it to generate cracks, but you can evaluate the Maximum Principal Stress and compare it with the Ultimate Tensile Strength to decide if a failure has occurred.  The Compressive Ultimate Strength value in the Material is only used by the Stress Tool to plot Factor of Safety.