You could use an integer to represent the value you want to vary, like this:

#declare VaryAmbient = 1;
#declare VaryDiffuse = 2;
#declare VarySpecular = 3;

#declare WhichValueToVary = VaryAmbient;

#declare Variable = ... // Vary this

// ...
finish {
  ambient (WhichValueToVary == VaryAmbient) ? Variable : 0.1;
  diffuse (WhichValueToVary == VaryDiffuse) ? Variable : 0.6;
  specular (WhichValueToVary == VarySpecular) ? Variable : 1.0;
}
// ...

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