Apparently, AMD was considering implementing Intel's Foveros 3D technology at some point, but later decided to develop a better proprietary Micro Bump 3D packaging that is 1 micron thinner and quite a bit more efficient. This is only the beginning, as AMD plans to refine the interconnect pitch in the future, allowing for more complex 3D stacking applications.
https://www.notebookcheck.net/AMD-presents-more-details-on-Zen-3-3D-V-Cache-and-the-future-of-3D-stacking.556527.0.html