I tried to find 3D lemmings level editor, but I couldn't find,
I never looked any 3D lemmings custom levels, so I guess that there's no level editor for 3D Lemmings.. But I want it!
I don't think so. You may need to search for one of the hackers around to request the editor for you. One of hackers, GuyPerfect has created the Lemmings 2 editor. You could ask him, but I don't think he's around anymore.
A quick search took me to
this page, where there is a link near the bottom to a text file about the Lemmings 3D level file format. Sadly the link doesn't work, but it does suggest that some progress has been made at some point towards figuring out the format (which of course is a key step in making a level editor). Lem3D hasn't received much (if any) attention in my time on the forums, so I haven't heard of any attempts to make an editor.
Any work that was done on it, as far as I'm aware, was done by me. I never really got much useful info though, and the biggest catch was that the PC full version levels (but not the demo or the PSX version levels) are compressed. I haven't seen if the uncompressed PSX levels work on either PC version.