We introduce a new combination of diene/diene microstructures to form microphase-separated ABC terpolymers. The synthesis and morphology of two triblock terpolymers of the poly(butadiene-b-styrene-b-isoprene) (BSI) type and one poly(styrene-b-butadiene-b-isoprene) (SBI) type are reported. The microstructure for the PB block was 92 wt % 1,4- and 8 wt % 1,2- and for the PI block 54 wt % 3,4-, 16 wt % 1,2-, and 30 wt % 1,4-. A three-component microphase-separated system occurred even when the two diene blocks were adjacent, as in the SBI sequence, leading to the conclusion that the PB:PI 1,4/3,4 diene combination has a larger Flory-Huggins chi parameter than the other three possible combinations of PB:PI diene micro structures. A new type of cylindrical microdomain structure with alternating types of diene regions along the cylinder axis is observed.