The current editor viewport does not yet have the love and attention to detail necessary for productive 3D level design, but it will likely come at some point. There are other options, such as using Blender as your 3D editor tool.
Check out this thread
There are also tools like https://trenchbroom.github.io/ which you can use to make levels for Defold projects. I believe there has some some discussion on using it either in the forum or the discord.
