Send polygon 3 to the ‘Clipper’ port of a Clipper transformer, and send polygons 1 and 2 (or just polygon 2 - use a Tester if needed) to the ‘Clippee’ port then keep the ‘Inside’ part of the result in the Clipper transformer’s output.
Editing - it seems the clipping approach may not do it because not all of the new boundary is overlapped by polygon 3. In that case using the align edge topology tool in ArcGIS Pro (for example) may be easiest.