3D games ARE possible in construct 2
And indeed, you need to create a plugin.
I tried, and I achieved result, however such plugin is a real pain to use because if you want to really work with C2's picking system, you need more than one plugin.
Why? Because for 3D rendering you need many different abstract objects.
You need objects like lights and meshes. You need what's called a scene graph to organize your objects in the scene (add and remove them). You need a camera object to control position and direction of view as well as field of view. You need a renderer object that allows you to connect a camera to a given scene graph.
You also need materials and texture. (I'm mostly refering to how three.js works but I've seen such design in some software as well)
And you can't really stick everything into one big fat plugin. Well... You can, but then you wouldn't be able to pick objects since everything would be handled by the plugin. You would also have to find a way to keep some references to instances to manipulate them in the event sheet...
So a better solution would be to have a renderer plugin, a mesh plugin, material plugin, etc, this way you could pick materials and assign them to meshes, pick light and add or remove from scene graph, etc
But that's a big work. And as some already said, you wouldn't even have anything showing up in the viewport of construct2.
And also you would have to learn how the plugin suite work... which would be almost the same as to learn how three.js works (and 3D rendering in general... mipmapping, geometry instancing, model view matrices, etc...)
And well... at some point... it would be like using a chainsaw to hammer a nail.
Use the right tool for the job.
I know c2's event system looks so easy that you feel this need to use it for 3D games, but remember, c2's event system is really just sugar coated programming. It wouldn't take that much effort to switch (for example) to unityscript (a bit more for unrealscript :D).
If you already have a good level in C2, you've got good basics of programming. So after probably a month or two with unity, you'll be able to make what you want.
And if you're good enough to create a 3D plugin for C2... Yeah... By all means, use unity :D
for curious people, my 3D tests with C2 looks like that:
Maze (use arrow keys)
Just a rotating object