The phase behavior of ABC triblock copolymer melts is studied using weak segregation theory and numerical self-consistent field theory (SCFT). We focus on nonfrustrated systems, with interaction parameters chi(AB) similar or equal to chi(BC) < chi(AC). Phase behavior of completely symmetric systems, with chi(AB) = chi(BC) and equal A and C volume fractions,f(A) = f(C), is studied over a range of values of chi(N)(AC) and the ratio k chi(AC)/chi(AB), in order to resolve sonic qualitative differences between the results of previous SCFT studies that were carried out at very different values of k. Studies of full composition triangles are presented for two models with parameters similar to those of poly(isoprene-styrene-ethylene oxide), using values of x(ij)(N) twice those used in our previous study of the same models. A stable alternating diamond phase, with interpenetrating A and C diamond networks, is found to be stable in symmetric systems in a small region near the ODT. The Fddd (O-70) phase of slightly asymmetric ABC triblocks is shown to remain stable well into the intermediate segregation regime.