Logically, to get a 3D image using your webcam, you have to record with more than one camera from different angles, or with the same one from different positions. All webcams record flat, in just 2 dimensions. So, getting a 3D image is a complicated job that needs a lot of judgement, skill, patience and time.
This is the reason a program like StereoPic exists, that lets you record in stereo with your webcam, focusing from two angles so that the result of the final video is in 3D.
StereoPic works with the majority of existing webcams.