<p>Because of the important role of the absorption heat pump in low-grade thermal energy utilization, this paper extends it to micro domain and performs a finite-time thermodynamic modelling for a three-heat-reservoir (THR) thermal Brownian heat pump with heat transfer effect by using an equivalent combined cycle method, which was applied for macro endoreversible THR heat pumps. The working principle and energy transformation rule are studied, and the coefficient of performance (COP) and heating load are derived. With a fixed overall thermal conductance of three heat exchangers, the maximal heating load is determined by optimizing thermal conductance distributions among three heat exchangers and barrier height, and the optimal working temperatures are also obtained. The impact of external heat transfer is elucidated to show the difference between this model and a non-equilibrium thermodynamic one. Results indicate that external heat transfer determines the energy transformation directly, and performance characteristics are closer to reality when external heat transfer is considered. The heating load has a maximal value about thermal conductance allocation ratios. About half the overall heat exchanger inventory needs to be assigned to the heat exchanger of the heating space for maximal heating load. When the cycle is with only heat transfer effect, the net particle numbers are zero, and the cycle fails to pump heat. The research results are expected to offer an idea for thermodynamic optimization and design of micro THR thermal Brownian heat pump devices.</p>