The triple-row is a wonderful, versatile instrument but you need a single-row to play traditional Cajun music. Each instrument has its own capabilities, limitations, and distinctive sound. You cannot substitute one for the other.
More often than not, professional Cajun musicians gig with two or three single-row boxes. In addition to the single-rows, those who play Zydeco and/or Creole music typically have one or more triple-rows. But that’s not about covering more keys.
Who do you want to emulate? If it’s Octa Clark you need the single-row. But if it’s Boozoo Chavez you’ll need both. For a crash course in what the instruments are capable of, go see how Horace Trahan (or Jeffrey Broussard) choose among their single- and triple-rows for different parts of their shows.
Enjoy!