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 slightly more complex requirements, and it will perfectly make sense not to implement anything beyond the scope of the already implemented test cases.
Nevertheless, your "cheap approach" has indeed one advantage which makes it less nonsensical than you might think: chances are much better you don't forget to add all of the important test cases. If you implement the three-value comparison at once, there is a certain probability that you might omit further test cases, since you are already in the mental state of "beeing done". If, however, you know your code is "not ready" yet, and you force yourself not to change it without further test cases, chances are much higher you actually will take the time and add those test cases.