3D Map Tool is basically an extended room editor for dealing with 3D in GMS2. You can load any object from your game into the editor, scale it and move it around in 3D, though you'll have to configure them to draw on the z axis first. In addition there are several 3D primitive objects you can create and modify like in older Game Maker versions but using vertex buffers instead of d3d.
Create simple 3D objects like cubes, cones, and spheres and change their properties in a few clicks.
You can select, manipulate, and copy multiple objects at once.
Saves changes directly to room files in your project.
Almost everything in the UI is built and modifiable in GMS2.
Designed to be user friendly and easy to implement into any project.
You can download a demo version here which has everything except the save function.
Layer bug fixed when saving.