How to write arcs directly in G-code?
Marlin has G2 (clockwise arc) and G3 (counterclockwise arc) commands that could be used to do this. You can find detailed documentation for the command here.

Basically, you can use


  G2 R1 X5 Y5


to draw a (clockwise) arc from the current position to $(X,Y)=(5,5)$ with a radius of $1$.

So, your rounded triangle could be drawn with 3 straight line moves and 3 arc moves. Figuring out the exact coordinates for each move would be a quite challenging geometry exercise, as you'd need to know where the straight line portion of each side ends and the rounded portion starts.