Experimental evidence has revealed superconductivity with a critical temperature, Tc, around 100 K in compressed solid phosphine, but theoretical studies have hitherto found no stable structure in any binary P-H system, leaving the characterization of the new superconductor unsettled. Here we present the findings of an advanced structure search and first-principles calculations unveiling the effect of Li as an electron donor that stabilizes the crystal structure and produces robust phonon-mediated superconductivity in the resulting Li-P-H compounds in wide ranges of stoichiometry and pressure. We showcase a trigonal LiP2H14 phase that reaches Tc of 169 K at 230 GPa and then decreases with rising pressure, which can be remedied by substituting Li with Be or Na, which considerably enhances Tc. These findings highlight the intricate and effective chemical tuning of stabilizing the crystal structure and enhancing the superconductivity in a distinct class of ternary hydrides, opening new avenues for designing and optimizing new high-Tc hydride superconductors.