If you're not looking to create a triangle with exact dimensions you can try this:
- draw your triangle with the Shape tool but make it smaller than what you intend it to be. You can also just scale down a clone (Ctrl+K) of your current triangle and color it differently so it's overlayed onto the original.
- switch to the Contour tool and enable Round join
- drag out a contour from your triangle and when it's big enough click the Inset path button

Edit:
I hadn't thought of this, but I suppose if you want to preserve the exact dimensions you can do the following:
- draw your triangle with the Shape tool
- switch to the contour tool and make sure Round join is enabled
- drag towards the center and let go when the distance between the innermost contour and the original shape is the desired radius of the final corners
- Copy or note down the Contour width which is displayed in the toolbar, we'll call this X
- click Inset path
- now drag out a contour from this smaller triangle and input X as the contour width
- Inset path
Done