  We study the steady state of a three-level system in contact with a
non-equilibrium environment, which is composed of two independent heat baths at
different temperatures. We derive a master equation to describe the
non-equilibrium process of the system. For the three level systems with two
dipole transitions, i.e., the $\Lambda$-type and V-type, we find that the
interferences of two transitions in a non-equilibrium environment can give rise
to non-vanishing steady quantum coherence, namely, there exist non-zero
off-diagonal terms in the steady state density matrix (in the energy
representation). Moreover, the non-vanishing off-diagonal terms increase with
the temperature difference of the two heat baths. Such interferences of the
transitions were usually omitted by secular approximation, for it was usually
believed that they only take effect in short time behavior and do not affect
the steady state. Here we show that, in non-equilibrium systems, such omission
would lead to the neglect of the steady quantum coherence.
