Composite image showing the analytic spiral model superimposed on the reconstructed N-band MYTHRA image of 3 Puppis. The logarithmic colour scale represents the observed intensity of the N-band image (normalised to unity and expressed in ADU), while the spiral model is overlaid as a semi-transparent structure representing the local gas surface density normalised to 20% contrast. The red dashed circle marks the inner dust rim (R_rim ≃ 5.7 mas), and the black dotted circle indicates the radius of the SE asymmetric structure (R_ref ≃ 16.71 mas) inferred from the L-band MYTHRA image. The curvature and radial extent of the two logarithmic spiral arms closely match the two asymmetries observed in the N-band image, located in the SE and NW regions of the dusty disc.