At the DESY-beamline P64* a new three-signal beam position monitor (BPM) was constructed and tested in 2020. The BPM is based on the working-principle of an Ionization Chamber with splitted electrodes and a 120°-symmetry. The chamber is filled with an inert gas, which is ionized in presence of a beam. The gas can be changed, and the absorption can be adjusted in dependency of the X-ray-energy. The 2D-position is calculated out of three signals by a multiple-linear regression, where the position

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... can be obtained by using a coordinate-transformation, similar to the Park-transformation, which is well-known in the field of drive control. Calibration factors have been evaluated in detail by using linear optimization algorithms including weighted residuals. The calculation is an inverse problem, which can be solved either by Simplex-algorithm or by Moore-Penrose-Pseudoinverse. The different results have been compared. Moreover, in order to validate the feasibility, calibration factors have been compared in regard to different beam sizes. Non-linearities are shown for a grid of 3x3 mm.