I could be facetious and say all you need is a web browser to find the answers, lol!
First you need the Aircraft and Vehicle SDK, the Mission Editing SDK and the Mission Builder SDK, all from the Microsoft
CFS3 Insider site.
Next you need gmax from
TurboSquid (free) to build models - aircraft, buildings, vehicles. You'll need a half-decent paint program which can handle layers and alpha channel for the artwork:
Paint.NET and
Gimp are free, Paintshop Pro is reasonably priced in payware (especially secondhand) and there's always Photoshop if you have the readies.
.AIR files can be created with AirWrench (payware) or the Flight Dynamics Workbook (free, if you have MS Excel) - both from Gerry Beckwith's
Mudpond site -
AAM (free) or
AirEd (free). Aircraft cfg files need ... Notepad and that's all! That said I like
XML Wrench (free) for editing the .XDP damage files.
Building airfields is Pat Pattle's speciality and you'll find more info from him in the downloads here at SOH. Sound files I know nothing about, sorry! Nor separate views on multiple monitors; that's something I think is possible in FSX but not in CFS3.
2D Panels are a feature of Flight Simulator 8, 9 and an option in FSX, but they're entirely missing from CFS3; the 3D virtual cockpit is all we get.
Hope this helps! I should also say you need a fair helping of free time and large quantities of blood, toil, tears and sweat...
(W.S. Churchill)
:EDIT: I'd also echo the advice from elsewhere to check out FSDeveloper