The CFS3 aircraft textures are stored in dds format and are located in the CFS3/aircraft/plane_xyz folder.
Textures usually follow a certain naming convention so you can easily recognize them,
for instance the base texture always uses a "_t" at the end of the name, the reflection level map an _r" and the specular level texture an "_s", like this:
p_51d_t.dds
p_51d_r.dds
The specular level and reflection level maps are basically greyscale copies of the base map.
The paintwork can be done in a paint program such as Photoshop or Paintshop Pro (both cost money)
You can in theory paint in MS Paint which comes shipped with every Windows operating system but it gives rather crude results.
Non of the above programs can open the dds textures by default,
you would either need DXTbmp (freeware) to convert the dds files to bmp prior to opening, and resaving to dds after editing
or you download the free Nvidia dds plugin which allows you to open the files in Photoshop or Paintshop Pro directly.
DXTbmp is available here:
http://www.fly.to/mwgfxThe Nvidia plugin is available here:
http://developer.nvidia.com/object/nv_texture_tools.html