The nearby LHS-1678 (TOI-696) system contains two confirmed planets and a wide-orbit, likely brown-dwarf companion, which orbit an M2 dwarf with a unique evolutionary history. The host star occupies a narrow "gap" in the Hertzsprung-Russell diagram lower main sequence, associated with the M-dwarf fully convective boundary and long-term luminosity fluctuations. This system is one of only about a dozen M-dwarf multiplanet systems to date that hosts an ultra-short-period planet (USP). Here we validate and characterize a third planet in the LHS-1678 system using TESS Cycle 1 and 3 data and a new ensemble of ground-based light curves. LHS-1678d is a 0.98{+/-}0.07R{Earth} planet in a 4.97day orbit, with an insolation flux of 9.1_-0.8_^+0.9^S{Earth}. These properties place it near 4:3 mean motion resonance with LHS-1678c and in company with LHS-1678c in the Venus zone. LHS 1678 c and d are also twins in size and predicted mass, making them a powerful duo for comparative exoplanet studies. LHS-1678d joins its siblings as another compelling candidate for atmospheric measurements with the JWST and mass measurements using high-precision radial velocity techniques. Additionally, USP LHS-1678b breaks the "peas-in-a-pod" trend in this system although additional planets could fill in the "pod" beyond its orbit. LHS-1678's unique combination of system properties and their relative rarity among the ubiquity of compact multiplanet systems around M-dwarfs makes the system a valuable benchmark for testing theories of planet formation and evolution.