Three-arm star block copolymers were successfully synthesized by combination of “core-first” and “coupling-onto” strategies. First, polystyrene 3-arm star polymers (PS-Br)3 with high bromine chain-end functionality were synthesized by atom transfer radical polymerization (ATRP) using the core-first method. After complete transformation of the bromo groups to azido groups by nucleophilic substitution with sodium azide, the product (PS-N3)3 was used for click reactions with alkyne-terminated poly(ethylene oxide) chains (PEO-alkyne) and produced 3-arm star block copolymers (PS-b-PEO)3 via the coupling-onto method. The efficiency of the coupling reaction was influenced by the preservation of chain-end functionalities in both (PS-N3)3 and PEO-alkyne precursors. The azido functionality in the (PS-N3)3 star precursors was determined as high as 97% by gradient polymer elution chromatography (GPEC) analysis of the coupling product between (PS-N3)3 and propargyl alcohol. During the coupling reactions between (PS-N3)3 and PEO-alkyne, the functionality of PEO-alkyne was ca. 100%. In the final coupling product, the molar fraction of the targeted (PS-b-PEO)3 star block polymers containing three block arms was ca. 85% and the fraction of the star polymers with two block arms and one homopolymer arm, (PS)-(PS-b-PEO)2, was ca. 11%.