Over the past two decades, the Ball-on-Three-Balls-test (B3B) has been increasingly used due to its low systematic error and simple execution. A limiting factor for a more wide-spread use of the B3B-test is the lack of an accurate analytical solution. This demands the use of numerical methods, such as Finite-Element-Analysis (FEA). In a recent work, FEA has been used to assess the influence of non-linear effects on the measured strength. To validate the utilized FEA-model, the experimental measurement of the specimen's deflection can be drawn on. Due to the design of the fixture for the B3B-test, common deflection-measurement methods are not straightforward. Therefore, X-ray tomography is employed in this paper to track the displacement of the load and support balls for thin plates of a high-strength glass and Ce-doped zirconia presenting transformation-induced plasticity. Furthermore, the ball displacement is also determined from two-dimensional radiographs and shows good agreement with FEA results.