It is known that unambiguous discrimination among non-orthogonal but linearly independent quantum states is possible with a certain probability of success. Here, we consider a variant of that problem. Instead of discriminating among all of the different states, we shall only discriminate between two subsets of them. In particular, for the case of three non-orthogonal states, {|ψ1 , |ψ2 , |ψ3 }, we show that the optimal strategy to distinguish |ψ1 from the set {|ψ2 , |ψ3 } has a higher success rate than if we wish to discriminate among all three states. Somewhat surprisingly, for unambiguous discrimination the subsets need not be linearly independent. A fully analytical solution is presented, and we also show how to construct generalized interferometers (multiports) which provide an optical implementation of the optimal strategy.