I'm not sure it's a minimum of 3 decks the ladder connects to. The upper part could just be for environmental controls and in the screencap from "The Naked Time" it looks like there is a seating grate for the environmental specialist.
If these controls are for artificial gravity (my favorite theory), then the shot from "Amok Time" (Deck 5) would indicate that the way up is a dead end (you'd still need to fix artificial gravity for Deck 4 above but no further up).
are going for 3D interior builds, as far as I'm aware. With my project I can only offer classic 2D, for now.