Quake3World

Hello,
I was trying to make a triangular pyramid for a corner of a room I'm making in GTKRadiant. Having some trouble, could someone please point me to a tutorial on how to make this shape. I tried the clipper tool and I feel like im slowly driving myself insane. I checked under curve in Radiant, couldn't find that shape.
Thanks

Brush Primitives Cone Sides: [3]

Can also be done with the clipper tool by 3-point clipping: https://www.youtube.com/watch?v=K-ovhLgwmMo

Small illustration how the clipping tool actually works: