While binary carbon nitrides have attracted significant attention, ternary C–N systems remain largely unexplored. Among these, no ternary alkali metal-C-N compounds have been investigated in laser-heated diamond anvil cells (LHDACs) to date, leaving their thermodynamically stable compounds and C-N species at high pressure unknown. In addition, recent experimental and theoretical studies suggest that the presence of a third metal elements can promote the formation of novel C-N anions. Motivated by this, Li-, Na-, and K-C-N systems were studied in LHDACs up to 44 GPa, providing a comprehensive picture of phase diagram across these systems.