Hi Diggit,
I have very particular ideas about housing so my comments might well be quite different from many others but I will say it comes with many years of experience and I never come across easier to maintain houses than those I use.
You don't mention material and this is very important just as important as the layout and general design. I think it's a good idea to use sheet material, rather than narrow board. Sheets such as plywood and as good a quality as possible, 15mm thick is good. This will help keep down red mite and make cleaning easy. If you do use narrow board make it tongue and groove. Put the structural batons on the outside if possible to ensure your floor is completely flat so you can just sweep out the spent litter with NO dirt traps.
I don't like the single perch. Houses are for roosting in and if you put nestboxes in, for laying. Perches are very important and their profile should be rounded on the edges. A single bar is inadequate. Your entire floor area should be a grill of roosting bars raised 100mm or so above the floor so that the birds walk on the bars which actually simulate tree branches. The droppings fall through the bars onto the flat floor which is covered with litter. The bars should be 50 - 70mm wide with similar gaps so that your birds never walk on a fouled floor. This is the single biggest mistake I see constantly in house design that the birds are allowed to walk and pick in the dirty litter. This is the reason many hobbyist keepers poo pick daily. With proper design this isn't necessary and I generally don't clean out my houses more often than once every 4 to 8 weeks and no poo picking is needed. I sprinkle new litter on the soiled surface and build it up and only renew it when it fills the space below. With 3 birds and the size you are planning you can easily only need to clean every 6 weeks if you wish. A light topping of fresh litter when needed and a sprinkling of a dry disinfecting powder every week is optional.
Personally I wouldn't bother with a dropping tray they become heavy when laden with litter & droppings, especially the size you are planning, and are usually flimsy or heavy and have a tendancy to spill or give you a face full of soiled litter if it's windy. When you have such a nice open access flat floor and you do as I suggest with the perches, sweeping out a fixed floor is easy. Use a heavy polythene sheet (damproof membrane is ideal) to cover the floor and you can easily gather up the litter like a bag and the floor will stay dry & clean. A complete clean and replacing the litter should take no more than 15 mins once every 4 to 6 weeks. That's the target, low maintenance and sanitary conditions too. I certainly wouldn't use a metal sheet as this will be very cold and attract condensation in winter. This would be a definite no no for me.
The roof is probably one of the most important constructional features. It must be watertight and I'd strongly recommend Onduline corrigated bitumen board. Light, long lasting and most importantly maintenance free. Use absolutely no felt which will be red mite heaven. Slope the roof away from the pophole so water is shed away from the entrance. Don't use hinged acess doors. Hinges aren't low maintenance they rust and sag with time and soon your beautiful draft free doors will have sizable gaps. The simple answer is a detachable wooden side panel. Two batons on the inside over laping the bottom to form pegs which fit into two square holes in the edge of the floor and a couple of turn buckle toggles available at any good hardware shop on the top will secure it in place. The roof should overlap the walls by at least 75mm. Make it double pitched, if your skills allow, so it can be steeper than a pent roof with a roosting bar on the apex. Otherwise with a shallow pent roof, they will poop all over it and it doesn't shed rain and snow as well. You won't need extra ventillation with a corrigated roof.
The pophole should be a vertical rise and fall type don't, whatever you do, have a hinged door or worse a side sliding door. There's no need for the opening to be bigger than absolutely necessary and an opening of 250mm wide by 300mm high is fine for most hybrid layers. A vertically sliding door is easy to automate won't get clogged with droppings and won't stick if you make the slide properly. Why fit it centrally? Why not to one side away from the nest boxes to cut down drafts.
I'd also have two or three nestboxes the size of house would easily house 8 hens eventually so why not plan in a bit of expansion. 330 mm square is ample size but don't have it so they can roost on top of them.
Having all the internals lift out is a good idea. The key thing to bare in mind is low maintenance, flat floors, lift off sides, no dirt traps, removable fittings and keep the hens away from their droppings and no food or water in the house. Use a covered entrance run to house the food & water with a duck board slatted floor mat on woodchip in front of the pophole to keep the entrance dry and clean so no mud gets trailed in and the eggs stay clean as they don't walk in their droppings either and you will have one excellent house.
Best of luck
HF