Here's what you need to do.
1. Enter BOUNDARY and click within the area that you want to extrude. You'll want it set to polyline.
This will create a closed polyline from the bounding geometry (way cool).
2. Enter EXTRUDE, select the polyline, and choose the MOde option. Set it to Solid if necessary.
3. Enter a value for the extrusion distance.
At some point you'll want to rotate a profile by 90 degrees (think drafting glass box views), for which you'll need to the UCS command with the ZAxis option to serve as an axis of rotation. Attached is a dwg that steps through the method from left to right.
Left side: top, front, right side created in 2D
Left middle: Polyline profiles created and rotated into place
Right middle: Profiles extruded through each other
Right side: Enter the INTERSECT command and select the three solids
4. (Optional) Sign up for Autodesk University (November in Las Vegas) and attend my class in 3D Solid Modeling Basics in AutoCAD. 🙂
Dieter
Dieter Schlaepfer
Principal Learning Experience Designer
Autodesk, Inc.
San Rafael, California