(a) Schematic figure of a three-terminal triple-QD thermoelectric heat engine. Three quantum dots with a single energy level E_i (i = L,R,P) are connected in series to three fermionic reservoirs i. The chemical potential and temperature of the reservoirs are μ_i and T_i, respectively. The constant Γ represents the coupling strength between the quantum dot and electrode. We consider in this set-up that the terminal L is connected to a hot bath and terminals R and P are connected to a cold bath. Besides, the whole thermoelectric system is connected to a circuit with three resistors (denoted as R_i, i = 1,2,3). These resistors are respectively connected to three electrodes. I_i represents the current flowing through each resistor. (b) Schematic figure of a three-terminal triple-QD refrigerator, where terminal L is connected to a cold bath and terminals R and P are connected to a hot bath. In this set-up, the terminal L can be cooled.