I'm not sure where you're getting that info. The linked source indicates the third mutation in question precedes the first two.
> In multiple lineages, the substitutions A119S and A119N preceded substitutions to 111 and 122 (Hemiptera, Hymenoptera, Diptera). In Drosophila, where we have the greatest phylogenetic resolution, A119S was established before substitutions to sites 111 and 122 in the evolutionary lineage leading to D. subobscura, which appears to be polymorphic with respect to CG-insensitivity.
> In multiple lineages, the substitutions A119S and A119N preceded substitutions to 111 and 122 (Hemiptera, Hymenoptera, Diptera). In Drosophila, where we have the greatest phylogenetic resolution, A119S was established before substitutions to sites 111 and 122 in the evolutionary lineage leading to D. subobscura, which appears to be polymorphic with respect to CG-insensitivity.