ArcGIS - you can view 3D AutoCAD files in ArcGIS without having to convert. You can also easily convert DXF or DWG into ESRI Geodatabase (native spatial data format) if you which to furter manipulate/analyze the various features contained in the model.
A word of caution; whereas ArcGIS is good at displaying 3D AutoCAD drawings you will not be able to edit the existing meshes (multipatches in ArcGIS). ArcGIS is extremely limited in terms of 3D modelling and editing, geared more towards geoprocessing. Other limitation is being able to easily add annotations in 3D space - this is possible not there is not a truly dynamic way of doing this in 3D. On the other hand, labeling in 2D space is excellent.
In order to view and analyze an existing 3D AutoCad model in ArcGIS you will need:
ArcGIS (Basic; formerly ArcView)
ArcGIS 3D Analyst Extension (sold separately)
The 3D Analyst extension (as well as other extensions) are typically installed with the main software (installer option) and are seamlessly integrated within the application framework.
There are 2 ways you can view 3D Autocad data in ArcGIS; ArcScene (a 3D viewer) and ArcMap (Plan View only) which is the main mapping application in ArcGIS