This does not work unless you know more about Q. You are essentially assuming that Q = f(a,b,c) = f(a,b,1-a-b) = g(a+b)
for some function g. In many cases this is false so you are forced to test Q for (a,b) ranging over a grid of points in a triangle in [0,1]x[0,1] (a+b<1)
So again you are...