Thanks. A real pain...
Like what there are tricks to achieve this:
https://github.com/Scirra/Construct-3-bugs/issues/5191
Coming to 3D for the first time in C3, I didn't think it would be so difficult or even impossible to accomplish such simple things.
For information, and for all intents and purposes to anyone reading this, this answer from Ashley helped me a lot (among other hacks):
https://github.com/Scirra/Construct-3-bugs/issues/5698
Natively, I keep hope, especially when I see this, which may one day be implemented to be usable (not in centuries):
It would also be really nice if the collision polygons were exploitable on each face.
On the other hand, when I read the following, and even if it is true that the request lacks details, I have a little less hope. It is, however, the most voted 3D feature:
https://construct3-21h2.ideas.aha.io/ideas/C321H2-I-96
Finally, the editor view is currently very limited too, it's really not easy to build the desired scenes and when sprites and 3D are mixed, it's depth hell, 3D hides the sprites even with separate layers. If at least we could adjust the transparency to see it a little clearer, but no. :(