that is because you currently are using Ngons, N being more than 4 corners.
You need to add more lines to your mesh.
Select two vertices (vertex) and press the “connect” button in editable poly or editable mesh mode.
Try to make sure that you dont have polygons with more than 4 corners.
also, you might want to create a UV map for the materials to apply properly.
Good luck!
You need to model without convex N-gons. At the moment when 3DSMax triangulates the mesh, it will do so in a way that creates trianges across the areas you’ve cut out because it doesn’t know any better.
This would be the correct way to model it:
And it would be even better if there was another vertical line in the middle, because having all those corners cramped towards one vertex is actually less optimal for dynamic light.
sadly I cant find the blog talking about it atm.
bit over the top one:
Personally id go for a mix between Dave’s one and my example for optimisation’s sake 
Thank you. How do I keep the unrwapping after correcting the geometry? Or do I have to unwrap again?
Most of the time it stays rather identical but occasionally it does happen that you need to unwrap it again. it shouldnt be that hard or timeconsuming though
It should stay the same as long as you’re only adding extra edges, and not moving around vertices.
