Hi John,
The simplest way is to create 3D cells, see the help topic "Creating and Editing Cells" for the basics.
In AECOsim you can also create Compound Cells, these combine 3D and 2D graphics along with a perforator. A Compound Cell can cut holes in walls or floors that they are placed in and display 2D geometry instead of the 3D when plan/section callouts are created, see the help topic "To create a compound cell".
Families and parts can also be applied to Compound Cells (or standard cells) to further control symbology as described in Family and Part on Compound Cell
Cells can also be added to the DataGroup for scheduling, see AECOsim Building Designer - Compound Cells and Metadata for an example of this.
We also have online training material available covering these topics.