I don't know about construct, but for a reference on the amount of polygon in the screen for good realtime performance are VERY low compared to the amount of polygon for rendered movies, for example:
-On Nintendo 64 and playstation 1, polygon count were around 100.000 triangles for background,
characters, 2D elements, etc. (a cube has 6 squares=12 triangles). A character used at max around 1.000 polygons.
-PS2, Gamecube, Xbox 1 worked with around 20 million triangles on everything. (Note that this already incredible amount of triangles are only statical graphics, so whenever animation+interaction+ physics kicked in, this dropped A LOT, I play with Blender 3D and I can testify about this) A character would use around 2.000~5.000 polygons.
What I'm trying to say with this? These consoles were designed specifically to handle 3D objects and as you can see they didn't use too many, Construct can handle 3D, but as far as I know it's not its focus so I would expect it to have a 3D performance WAY below than any other 3D game/engine, despite using DirectX.
I haven't tested yet, but I think It should be safe to work with 3D models of 1.000 triangles app.