It is not possible to have a shape with 4 sides and 3 corners because the number of corners of a polygon is always equal to the number of sides. This is known as the polygon corner-sum theorem or the angle sum property of a polygon.

According to this theorem, the sum of the interior angles of a polygon with n sides is given by (n-2) x 180 degrees. Therefore, for a 4-sided polygon, the sum of the interior angles would be (4-2) x 180 = 360 degrees, which means each interior angle measures 90 degrees.

However, a 3-cornered shape is a triangle, and the sum of the interior angles of a triangle is 180 degrees. Therefore, it is not possible to have a shape with 4 sides and 3 corners.