ABS (AutoCAD Building Systems) is 100% completely 3D. it has it's own package dedicated for 3D work, and it's VERY nice indeed. you really don't have to do much work at all. AutoCAD is still the "core" application so the vanilla workspace is still there. it's geared strictly towards HVAC/Plumbing/Piping/General Mechanical design, etc etc.
check out your local reseller for a demo, i don't think they have ABS demo for download.