For the purpose of achieving ABC-type periodic terpolymers, various vinyl ethers (VEs), oxiranes, and ketones were examined in the cationic vinyl-addition, ring-opening, and carbonyl-addition terpolymerization and suitable reaction conditions were identified. When the appropriate monomers are employed, the terpolymerization proceeds via a one-way cycle of crossover reactions, similar to "rock-paper-scissors". Homopropagation reactions of VE and oxirane need to be completely suppressed while attaining highly selective crossover reactions to achieve ABC-type periodic sequence control. By screening various monomers, VEs and oxiranes with moderate but sufficiently high reactivities were found to be appropriate for both the frequent crossover reactions and the polymerization itself. Appropriate ketones were also important for achieving efficient terpolymerization. In addition, more frequent crossover reactions occurred in the terpolymerizations conducted in less polar solvents at lower temperatures. Under the optimized conditions, the cationic terpolymerization of ethyl VE, 4-vinyl-1,2-cyclohexene oxide, and methyl ethyl ketone proceeded via frequent crossover reactions, and this process yielded an ABC pseudo-periodic terpolymer with averages of 1.2, 1.4, and 1.0 monomer units per block, respectively.