It has been reported that the self-assembly of block copolymers (BCP) with very high molecular weight (MW) can achieve the length scale above 100 nm, which can be utilized as photonic band-gap materials or photonic crystals. However, due to slow chain dynamics, it is hard to fabricate well-controlled nano-patterns from high MW BCPs via thermal annealing process. In this work, we designed a new type of BCP, namely ARB type triBCP, where the R represents the short middle block composed of A and B random copolymer. It was expected that the R block increases effective dispersity via compositional distribution, leading to an increased domain size compared to the AB diBCP with same MW and dispersity. We prepared various ARB type triBCPs and AB diBCPs having the similar dispersity via living-radical polymerization, and their morphologies were characterized by TEM, SAXS, and GISAXS. Consequently, it was shown that the ARB-type triBCP exhibited ~50 % increase in the domain spacing compared to the AB diBCPs with same MW and dispersity. These results were also compared with theoretical prediction.