—Alkaline chlorides are important constituents of carbonatitic inclusions in magmatic minerals from kimberlites and lamproites, mantle xenoliths from kimberlites, and diamonds from kimberlites and placers around the world. This indicates the participation of alkali chlorides, along with carbonates, in the processes of melting of mantle rocks, which makes it important to study chloride–carbonate systems at mantle pressures. In this work, we studied the phase relations in the NaCl–CaCO3–MgCO3 system at 3 GPa in the range of 800–1300 °С using a multianvil press. It has been found that the NaCl–CaCO3 and NaCl–MgCO3 binaries have the eutectic type of T–X diagram. The halite–calcite eutectic is situated at 1050 °C and Na2# = 36, while the halite–magnesite eutectic is located at 1190 °C and Na2# = 77, where Na2# = 2NaCl/(2NaCl + CaCO3 + MgCO3) · 100 mol.%. In the NaCl–CaCO3–MgCO3 ternary, subsolidus assemblages are represented by halite and calcium–magnesium carbonates. Just below solidus, two assemblages are stable: halite + magnesite + dolomite and halite + dolomite–calcite solid solution. The minimum on the liquidus/solidus surface corresponds to the halite–Ca0.84Mg0.16CO3 dolomite eutectic, located at about 1000 °С with Na2#/Ca# = 34/84, where Ca# = Ca/(Ca + Mg) · 100 mol.%. At Ca# ≤ 73, the melting is controlled by the halite + dolomite = magnesite + liquid ternary peritectic, located at 1050 °C with Na2#/Ca# = 31/73. According to the data obtained, it can be assumed that at 3 GPa the solidi of NaCl-bearing carbonated peridotite and eclogite are controlled by the peritectic reaction halite + dolomite = magnesite + liquid, located at about 1050 °C. The melting is accompanied by the formation of a chloride–carbonate melt containing (wt.%): NaCl (35), CaCO3 (56), and MgCO3 (9).