I agree with Bernie that chimney type kettles are ideal for boiling water, I have a large Ghillie with cooking attachments (I don't get on with using it to cook on, it needs to be full of water....ask Mr Fish
)
For just a brew my "Mkettle" is great and my most grabbed choice, it's very quick,cleanish and smells divine
. (and it will work in pouring rain)
But for cooking I regularly use one of two choices my wild woodgas stove or Nimblewill 6" type (which many of you would have seen my take on it's build/design on this forum) Both make excellent wood stoves and also wind shields for a meths burner, my nimblewill nearly fits in a vinyl CD wallet ( it goes in but is difficult to get out )
I would agree with Old Dog and Bernie that 3" stoves would be very difficult to manage, I've tried homemade gasifiers and they will burn out if not carefully watched and tended