Not a hope of going without a door unless the head is on the 36" wall. Even that is narrow and would benefit from a glass screen.

Nope, you need a door. Putting doors in my 4' x 4' shower and my 5' x 5' shower, but not in my 7' x 4' or my 8' x 4' (I'm considering them "galley showers" since they are long and narrow and the user can enter from the end opposite the showerhead.

maybe.

are 2 long sides and one short one going to be enclosed, with the open door on the other short side? i'd still put your shower head closer to the far (narrow) wall. but you'd feel pretty trapped in there, 3' isnt very wide, you dont have a lot of room to back up further from the spray. mount the head pretty high, so its more down on your than out on you.

any other set up than this 'tunnel fashion' isnt going to work.

dont worry about being cold, a combination heater/vent in the ceiling works fine. our open shower is 5x8 and its fine all winter with the heater on.

Think of the galley showers as long rectangles. For the showers, they are curbless showers which drain "to the front" near the showerheads. One long wall and both short/end walls are tiled to the ceiling. The other long wall has a 3' opening for entering, but the remainder is walled/tiled to 56" and then have glass to the ceiling for lots of light.

katya, you should put a tape line in your existing tub/shower at the 3' mark, and see how it works for you... no stepping back/over! that will give you a sense of whether your shower head can be high enough/ at a steep enough angle down, for you to comfortably shower.