There are two ways to create a triangle cabinet under the staircase. (Youtube video at the bottom).
Use the Staircase with cabinet in the public model library.
2. Draw a customized cabinet in Custom design>Closet.
Tutorial
Go to the Custom design> Closet.
2. Find the Cabinet
3. Drag Staircase(Cabinet) into floorplan and adjust size.
4. Detailed parameters can be adjusted, such as the number of doors, the size of each door, and so on.
5. Finally, you can check the rendering results:
Here is is the youtube video for your reference: