Join Vector3 with BoxBuffer Geometry to make a proper angle join using Threejs

Question

I want to create a view which has 6 sides mentioned in the image respectively, we have created all sides and joined respectively but we are stuck at a point when we need to create a side with some angle and need to join with the horizontal side

We need to create #3 and join with #2 and #4 in such a way that it should be error free we are open for suggestion.

Thanks in advance