  Three magnetic relativistic Schr\"odinger operators are considered,
corresponding to the classical relativistic Hamiltonian symbol with both
magnetic vector and electric scalar potentials. Path integral representations
for the solutions of their respective imaginary-time relativistic Schr\"odinger
equations, i.e. heat equations are given in two ways. The one is by means of
the probability path space measure coming from the L\'evy process concerned,
and the other is through time-sliced approximation with Chernoff's theorem.
