Multi-level cell spin-transfer torque random access memory (MLC STT-RAM) demonstrates great potentials in onchip cache design for its high storage density and non-volatility but also suffers from the degraded access time, reliability and energy efficiency. The existing MLC STT-RAM cache designs primarily focus on the performance and energy optimizations, however, often ignore the crucial demand for reliability. In this work, we propose a tri-region MLC STT-RAM cache design (TMSC) to simultaneously meet the requirements of performance, energy, and reliability. The tri-region MLC STT-RAM cache is optimized partitioned into fast, mixed, and slow ways according to different access performance, energy and reliability. A new error correction code (ECC) scheme, namely, non-uniform strength ECC (NUS-ECC), is also developed to tolerate the different bit failure rates in these ways. Compared to the latest performance-driven MLC STT-RAM cache design with pessimistic ECC scheme, our TMSC technique can improve the system performance and energy by averagely 9.3% and 9.4%, respectively, for various applications. The additional area cost associated with NUS-ECC is limited by 3.2% compared to the pessimistic ECC scheme.