We report differences in the thermodynamic behavior of compositionally symmetric ABC and BAC triblock copolymers near the order-disorder transition temperature and of binary ABC/BAC copolymer blends: poly(styrene-b-isoprene-b-dimethylsiloxane) (SID) and ISD with segment volume fractions near 0.33 and similar molecular weight disorder at different temperatures and from different ordered-state symmetries. Blends of symmetric SID and ISD molecules with molecular weights near 10K are miscible and form a gyroid morphology over a wide range of blend compositions. Flory-Huggins interaction parameters for this ABC system allow a detailed interpretation of SAXS, SANS, and DMS measurements. Symmetric SID disorders from the lamellar state, while ISD disorders from hexagonally packed cylinders of D blocks in a mixed matrix of S and I blocks. Block mixing occurs to reduce the number of S-D interactions forced by chemical connectivity in ISD. This drive to minimize unfavorable interactions is the underlying cause of the results outlined in this report.