Open a map in serious editor, press p button.
Select all polygons you want to export, it can be a piece of a map, or the whole map (don't use ctrl+a, because it's selecting sectors too, and it's shitty).
When selected, press file>export 3d object, and name it as you wish.
You'll need program to open it, it's name is Milkshape 3D downloadable here: https://chumbalum.swissquake.ch/ms3d/download.html
install it, and open.
Press file>import>RAW, and find your exported file from serious editor.
Voila, it's your map here as a model.
You can file>export, and choose best fit for you (for ex .3ds to open in 3dsmax studio).
Screens tomorrow maybe