Paul, there really isnt much to learn in the way of buildings in FSDS ( any version ) as most are simple boxes with the texture used to create an impression of detail.
An over complicated object in FSDS or even Gmax will impact on FPS .
You would not model a Door with hinges or the door knob and panels as it would be a waste of processing power, therefore, your door would be a plain polygon with the image of a door ( texture ) to give the impression of detail.
The most common error i see when users of either Gmax or FSDS creat a building is the scale is incorrect. As a builder by trade, im lucky that i can visualise scale as certain quanteties are known ( Door sizes ) Window head heights etc so i am able to approximate sizes if i dont have a scale plan to work from.
In Australia, most Doors have a height of 2.1 meters and the tops of windows are normally 2.1 meters aswell. most doors used as a main entrance can safely be set at a width of 0.9 meters as this would be close enough.
If you had an image of the front of a house, with the door at approx 0.9 meters width, using a set of dividers, you could scale out the width of the building to a fair approximation if you didnt know its size.
If this is confusing , let me know and ill try to explain or create a tutorial for you after Xmas as im very busy at work at the moment.
FSDS3 comes also with a few sample files (although most are aircraft). The help files do contain an introduction to making simple scenery objects, but I am not sure if they have all been updated from FSDS2 to FSDS3 already. Together with forums like this, you should be able to learn FSDS3 without too much trouble though.