1. **Problem Statement:** A plane cuts through a rectangular pyramid passing through three of its lateral faces and is perpendicular to the base. We need to determine the shape of the cross section formed by this cut. 2. **Understanding the Pyramid and the Cut:** A rectangular pyramid has a rectangular base and four triangular lateral faces. The plane intersects three of these lateral faces and is perpendicular to the base. 3. **Key Geometric Fact:** When a plane cuts through a pyramid and is perpendicular to the base, the cross section formed is a polygon whose vertices lie on the edges of the pyramid where the plane intersects. 4. **Analyzing the Intersection:** Since the plane passes through three lateral faces, it intersects three edges of the pyramid (each lateral face shares an edge with the base and the apex). The intersection points on these three edges form the vertices of the cross section. 5. **Shape of the Cross Section:** With three intersection points, the cross section is a triangle. This is consistent with the image description where the cross section is triangle-shaped. 6. **Conclusion:** The cross section formed by the plane cutting through three lateral faces of the rectangular pyramid, perpendicular to the base, is a triangle. **Final answer:** The shape of the cross section is a triangle.