One frame, use all 3 bits on. Means if popping into the shop can just put the car seat onto the wheels for example.
So different from when I had my 4 all those years ago. We had a carrycot that you could put on wheels, then we got a McClaren lie back pushchair, one of the first with swivel wheels, cost a fortune back in them days.
I could go on, but this isn't the chat place for it, would need a different post. But I used Terry towelling nappies for all 4 of mine, and disposable nappies before they had elasticated legs!