I think 3 is best for the long-term narrative construction. And one more thing:

Five implies a sentai show.

Right, but I was surprised to learn from Sunred that you can have a sentai group of 3 (e.g. Red himself as Weather Red, Weather Blue, and Weather Yellow).

BTW, Sunred 13 offers a great relationship advice. It works IRL. Due to its practical nature it's a rare episode that I suggest watching with subtitles, unless you're fully fluent.