Astrometric data of Gaia BH3. Top-left panel: Motion on the sky of the photocentre of the source, as seen by Gaia in the different CCD transits (dots), compared with the best fitting single-star solution from AGIS and the astrometric-binary solution from the NSS pipeline; the arrow indicates the direction of the proper motion. Bottom-left panel: Derived astrometric orbit of the photocentre, after a subtraction of parallax and proper motion, compared with the astrometric measurements. We note that only one-dimensional (1D) along-scan (AL) astrometry was used by the NSS pipeline. The position of the photocentre on the sky corresponding to each measurement is derived combining the measured one-dimensional AL position and the assumed orbital solution. The + signs show the barycentre and the position of the periastron, the dotted line shows the line of nodes, and the arrow indicates the direction of the motion along the orbit. In the top-right and bottom-right panels, we can see the residuals of the along-scan (AL) astrometric measurements for, respectively, the single-star solution and the binary-star solution. The vertical dot-dashed line in the bottom-right panel marks the time of the periastron passage.