Hi,
You would just mount the third partition in your fstab.
We discuss this approach in the OTA webinar (Over The Air Updates for Embedded Linux Devices) and it’s implemented in our meta-memfault-example layer for Yocto (https://github.com/memfault/memfault-linux-sdk/blob/kirkstone/meta-memfault-example/recipes-core/base-files/files/qemuall/fstab.append).
best,
thomas