The published weights probably don't tell the whole story, because one engine type may not include certain QEC gear that needs fitted before the airplane is ready to fly. I don't have details handy, perhaps someone can fill the void so we are comparing apples to apples.
Offhand I'd expect the three-shaft engine to be a bit heavier (D'OH! One more heavy shaft...) plus more bearings, oil system, etc. but this could quickly be repaid in reduced fuel burn.