This isn't an image is it? You actually have 1D signals in arrays that you plotted, right? Like
plot(signal1Red, 'r-');
hold on;
plot(signal2Cyan, 'b-');
plot(signal3Pink, '-', 'Color', [1 .8 .8]);
So in that case, I don't know what you mean by extract curve 3 by using all three curves. You have the signal in an array so you already have it "extracted" before you even plot it.
If you mean that you have a paper with those plots on it and you need to turn it into arrays, then you can scan in the paper and use this: http://www.mathworks.com/matlabcentral/fileexchange/36904-matlab-script-for-digitizing-a-published-graph
