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

[Rongway]

I don't see why continuation can't just be a passive trait that replaces the respective spot in the combo instead of a button by itself.

You mean like 1(1)-1(1)-1(1)? That would make it impossible to do it wrong. You can currently forget to use Continuation, or, because Continuation seems a bit finicky on its queuing, your Continuation press might not register when you think it does. The combo itself should definitely just be 1-1-1, but Continuation should stay its own button so you can still fail to press it. 1(2)-1(2)-1(2)

Edit: It occurs to me in hindsight that maybe you meant 1(1)-2(2)-3(3). Yes, this would also be better than 1(4)-2(4)-3(4). I think 1(2)-1(2)-1(2) would be best though.