Interesting idea. This should be possible, probably with non-linear constraint option.
However, (disclaimer: this is just a thought/curiosity, no guarantees at all), what if you just sort x at the beginning of my_fun()? This would enforce the constraint in your fitness function and possibly cause convergence of the guesses. Let me know if/how it works for you.
function z=my_fun(x)
x = sort(x);
a=2;
b=3;
c=1;
z=a*x(1)-b*x(2)+c*x(3);
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