To exploit the full potential of advanced high-strength steels (AHSS), a more in-depth understanding of the complex micromechanical interaction of thin-film retained austenite (RA) and lath martensite is indispensable. Inspired by the medium-Mn steel microstructure, a three-dimensional micromechanical modeling approach is therefore proposed in the present work, embedding the thin RA films explicitly into the hierarchical lath martensite structure. This enables systematic studies of the effect of RA film thickness and volume fraction on the local stresses and strains as well as their partitioning within the microstructure. The investigations reveal that with shrinking RA volume fraction, both stress and especially strain heterogeneity in the thin-film RA intensifies. In the martensite blocks, stress and strain heterogeneity also intensifies, although stresses are generally more heterogeneously, and strains much more homogeneously, distributed than in RA. The results underline the key role of RA with thin-film morphology for further optimizing AHSS microstructures.