A solution can be :)
Array curve a point to the ellipse
--> length base curvy triangle
create a cone 3D with radius = 1/2 length base triangle
Array curve cone to the ellipse
Extrude ellipse
Trim cones with extruded ellipse
et voilà
triangular surfaces arc-elliptic are equal ! ;)
If you want real straight triangular : polylining the 3 vertex of the triangular surface arc-elliptic :)