Hi @Lisha
You will need to add all source files for all cores into one project. Unfortunately, this cannot be done automatically, so you will have to copy these files manually.
Alternatively, you can create binaries for each .elf file and merge them. However, raw binary files do not contain address information, which might complicate the process. A better approach would be to generate three S-record files and merge them using the SRecord tools, which can be found here: SRecord
For a related example, there is a multi-core example for a different device (MPC5777C) on the community. This might serve as a useful reference: Example MPC5777C Single ELF multi-core Pin Toggling - S32DS Power 2017.R1
Although the example is for a different device, it could still be valuable as a reference for your setup.
BR, VaneB