I am trying to plot a function in *Mathematica* that is defined over the unit simplex. To take a random example, suppose I want to plot sin(x1\*x2\*x3) over all x1, x2, x3 such that x1, x2, x3 >= 0 and x1 + x2 + x3 = 1.
Is there a neat way of doing so, other than the obvious way of writing something like

```
Plot3D[If[x+y<=1,Sin[x y(1-x-y)]],{x,0,1},{y,0,1}]

```

![Mathematica graphics](https://i.stack.imgur.com/OLs4f.png)
?

What I want, ideally, is a way of plotting **only** over the simplex. I found the website <http://octavia.zoology.washington.edu/Mathematica/> which has an old package, but it doesn't work on my up-to-date version of *Mathematica*.