[en] A theory of hot electron cooling power due to polar optical phonons P _op is developed in 3D Dirac semimetal (3DDS) Cd₃As₂ taking account of hot phonon effect. Hot phonon distribution N _q and P _op are investigated as a function of electron temperature T _e, electron density n _e, and phonon relaxation time . It is found that P _op increases rapidly (slowly) with T _e at lower (higher) temperature regime. Whereas, P _op is weakly decreasing with increasing n _e. The results are compared with those for three-dimensional electron gas (3DEG) in Cd₃As₂ semiconductor. Hot phonon effect is found to reduce P _op considerably and it is stronger in 3DDS Cd₃As₂ than in Cd₃As₂ semiconductor. P _op is also compared with the hot electron cooling power due to acoustic phonons P _ac. We find that a crossover takes place from P _ac dominated cooling at low T _e to P _op dominated cooling at higher T _e. The temperature at which this crossover occurs shifts towards higher values with the increase of n _e. Also, hot electron energy relaxation time is discussed. It is suggested that can be tuned to achieve faster or slower energy loss for suitable applications of Cd₃As₂. (paper)