Partitioning kernels often face challenges such as static resource allocation and insufficient temporal protection, limiting their applicability in dynamic and mixed-criticality systems where resource needs and security boundaries evolve over time. To address these limitations, we present S3K, a capability-based multicore partitioning kernel for embedded RISC-V systems. S3K provides robust spatial and temporal isolation, time protection, and dynamic resource reconfiguration, enabling flexible adaptation to changing operational requirements while maintaining strong safety and security guarantees. Its capability-based model ensures secure and efficient resource management, while in-kernel data partitioning prevents information leakage and mitigates side-channel attacks. Additionally, S3K's scheduler guarantees deterministic process dispatch, free from microarchitectural interference. Evaluation results demonstrate S3K's effectiveness, showing the absence of scheduling jitter, resistance to intra-core side-channels, and efficient inter-process communication. These results highlight S3K's suitability for safety-critical and security-critical applications in dynamic, resource-constrained environments.