What is suggested way to download 3D symblos to TurbocadDeluxe17?

Were you planning on visiting the TurboCAD website and downloading one of their symbol bundles?  There is a cost associated with this.

No. I have them.  Like to download from cad studio or other sites.  Specifically interested in gears, chains, sprockets, etc.

So what is the problem then?  You don't know where to place the symbols?

May be it won't help but in AutoCAD Mechanical we have 2D libraries with the following commands :
AMCHAINDRAW, AMCHAINLENGTHCAL, AMROLBEAR2D, AMSPROCKET, etc...

If you have already purchased and downloaded symbols from the TurboCAD website then you must have some idea of the folder structure: name of folder and location.  I would look to emulate that same structure so when it comes time to insert one of these symbols TurboCAD has no problem finding it.