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I think your problem arises only because the requirement is very simple and the solution comparing all 3 values at once maybe just a one-liner. Having a slightly more complex requirements, and it will perfectly make sense not to implement anything beyond the scope of the already implemented test cases.

Nevertheless, you "cheap approach" has indeed one advantage which makes it less nonsensical than you think: chances are much better you don't forget to add all of the important test cases.