DescentBB

Does anybody know of any manuals or tutorials on how to use the D3 level editor? I want to make my own levels, but I don't now how it works

You will probably have to use the Wayback Machine ( https://archive.org/ ) to access all of these:

Official D3Edit Homepage
http://www.d3edit.tricordnet.com/tutorials.shtml

Robo's D3edit Tutorials
http://www.planetdescent.com/d3edit/

D3Edit Tutorial Zone
http://www.gameedit.com/hydra/

Papacat's D3Edit Training Camp
http://www.gameedit.com/papacat
http://www.gameedit.net/custom/GameEdit_OG/papacat/

Nightmare's Descent 3 Editing Tutorials
http://www.gameedit.net/custom/GameEdit_OG/index.html

Archived Descent 3 Editing Tutorials (from gameedit.com)
http://www.schplurg.com/gameedit/

Thank you for the help!