That's a good question and this is the right time to ask it -- a lot of people are not as foresighted
16" is not a lot of space. I can tell you my experiences with my original tractor (now departed, but its page is still up, go to the coop design tractor pages or just follow the link from my personal page). It was 2x4', with the roost just past the midline along the long axis so not *much* less free space than you have, and the roost was I think 16-18" off the floor? (It may actually say on my tractor page, I regret to say I'm too lazy to look
). Anyhow, the chickens *invariably* used the roof of the nestbox as a step to get up to the roost. I mean, that is what I put it there FOR of course, but I do not believe they ever went directly up or directly down *without* using that as a step. This would lead me to believe that the space provided in my tractor, with the roost at that height, was too narrow to permit them to get up/down directly (at least not comfortably).
From that, I would venture to guess that if you have 16" of horizontal free space, you probably would not want your roost more than a foot or so up.
Unless it changes other design decisions, though, what if you just attach your roost in such a way that it is easily repositioned -- then start it at a plausible altitude, and if it turns out not to work so great, you can easily lower it.
Of course, you could buy yourself a bit more space if you were able to run the roost the other way, making it just 3' long which is plenty for only three hens... that way it could be like 16-18" from the wall and still have something like 2.5' of free space in front of it. This might put the feeder and waterer in the flightpath of hens flapping down off the roost, which is generally not such a good thing and leads to collisions and spillage... except that if your coop is going to be high enough that you could put a wide droppings board under the roost and the feeder/waterer under THAT, then you might have a pretty good arrangement there. Something to consider anyhow.
Good luck, have fun,
Pat