I'd like to make a RegionPlot on a ternary diagram, meaning in a triangle where the axis are $x,y,z$, which are related by $x+y+z=1$. The RegionPlot would have as a condition $x^2+y^2<1$ for example.
These are examples of ternary diagram : https://www.google.com/search?q=ternary+diagram&safe=active&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiv37DH_-7bAhWBGhQKHTPjCYMQ_AUICigB&biw=1920&bih=1089