... Some of the serious payware developers use original manufacturers' blueprints to build accurate 3-views from: that is another level of seriousness altogether! ...
But much easier to use as a basis for your aircraft!!
I usually spent more time researching, measuring, comparing, extrapolating etc. than modeling itself (and then I still keep redoing parts after coming across a better reference). I would have loved to use real blueprints but the set I came across was about 5000,- ($ or €, can't remember). So you either
have to go for payware (not viable for CFS3) or be seriously deep into this thing...
There are/were some good 3-view artists around but I found that even they make mistakes or combine characteristics of different versions of the aircraft of choice. So collecting plenty of reference material is always a good thing. The aforementioned mr. Bentley has a good rep. Wylam and Paul Matt also come to mind. There are some Eastern European book series on aircraft that seem to have pretty good scale drawings inside that may serve as a backdrop. Most of them seem to be Polish or Czech. Scans can be found on the internet but unfortunately they are not always of usable quality.
Pilot Flight handbooks and Erection and Maintenance manuals may also contain dimensions and exploded views of parts etc. Collect all the info you can and store them in a file, including the webaddresses where you found the info (in case you want to revisit the site. It has happened more than often that you can't remember where you found the info).
If you have a specific aircraft in mind, I could have a look in the developer's library and see what we have.