plot angle in a triangle

How can I plot an graphically angle in a triangle (between two sides) in my figure?

[darova]

Is the triangle always flat? I mean 2D? Or it can be 3D

Answer 1

[KSSV]

Refer this:

https://in.mathworks.com/matlabcentral/answers/442706-how-to-draw-a-arc-with-arc-length-and-angle

Answer 2

[Rebecca Schoor]

i mean like in the pic: e.g alpha