So why the GNB cartridge combo need 3 different bottoms for its GCD?

[Rongway]

I wish they'd add the option to allow people to customize combos like this.

This is a special case though. The cartridge combo can never be used out of order, and can never be restarted once in progress. Therefore all three steps should be the same button. The same argument can't be made for most other combos.