How to draw Fill Triangle using the library Adafruit GFX library?

Other Hardware Displays
2

First off. Make sure that your libraries are up to date via the Library Manager.

I would delete any libraries that came on your CD. Only use libraries from the Library Manager.
i.e. Adafruit_GFX, MCUFRIEND_kbv, TouchScreen

The fillTriangle() method should work. Please post your sketch. I suspect that you just have a typo somewhere.

David.