  Quantum thermal devices which can manage heat as their electronic analogues
for the electronic currents have attracted increasing attention. Here a
three-terminal quantum thermal device is designed by three coupling qubits
interacting with three heat baths with different temperatures. Based on the
steady-state behavior solved from the dynamics of this system, it is
demonstrated that such a device integrates multiple interesting thermodynamic
functions. It can serve as a heat current transistor to use the weak heat
current at one terminal to effectively amplify the currents through the other
two terminals, to continuously modulate them ranging in a large amplitude, and
even to switch on/off the heat currents. It is also found that the three
currents are not sensitive to the fluctuation of the temperature at the low
temperature terminal, so it can behave as a thermal stabilizer. In addition, we
can utilize one terminal temperature to ideally turn off the heat current at
any one terminal and to allow the heat currents through the other two
terminals, so it can be used as a thermal valve. Finally, we illustrate that
this thermal device can control the heat currents to flow unidirectionally, so
it has the function as a thermal rectifier.
