An oligo(carbonate-ether) triol has been synthesized by the copolymerization of propylene oxide (PO) and CO₂ using a zinc–cobalt double metal cyanide (Zn/Co DMC) catalyst in the presence of trifunctional 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) as an initiator. The number-average molecular weight (M_n) of the polymer is controllable between 2300 and 5700 g mol⁻¹ simply by adjusting the amount of THPE. The carbonate unit content of the resultant polymers is tunable between 0% and 42.2% by altering the CO₂ pressure. The highest catalytic activities to produce an oligo(carbonate-ether) triol and an oligopolyether triol reach 4.37 kg g⁻¹ and 5.38 kg g⁻¹, respectively. In addition, the weight fraction of the cyclic propylene carbonate (W_cPC) byproduct is less than 5.4%, which is the lowest W_cPC level for the synthesis of branched polyols ever reported. The cyclic propylene carbonate moieties are mainly formed in the early and later stages of the reaction and the rapid propagation leads to suppression of the formation of cyclic propylene carbonate moieties. The evolution of the number and length of multi-arm polymers was investigated by NMR and ESI-MS analyses, which show that there are three distinct growth stages. A plausible mechanism of DMC catalyst-mediated copolymerization has also been proposed. This work demonstrates an efficient pathway to fix CO₂ into multi-arm polycarbonate polyols.