Since the Constructicons were numerous clones, the ones we saw forming Devastator didn't take on robot modes before they merged - and maybe they sacrificed (or forced to sacrifice) their individual robot modes to be able to merge with each other to create a combined mode.
Of the Legends set and this large Devastator set, this one is probably more movie accurate because there is no proof that the ones forming Devastator still had individual robot-modes (we just assume they did, because other Constructicons that didn't form Devastator, and probably can't, had robot modes). Magmatron in the JP Beast Wars series is the same, in that 3 beasts with no individual robot modes, merged to form the combined robot mode of Magmatron.