Q3Map2 is an updated version of the compiling program used by Id Software to convert MAP files to BSP's as well as calculating lightmap data, surface data, collisions, portals, etc. Since Id Software released the source code for the original Q3Map, ydnar has gone through great lengths into improving it. As a matter of fact, most recent Q3-engine based games were compiled with Q3Map2 as opposed to the original Q3Map (Jedi Academy, Enemy Territory, for example).
Recent versions of GtkRadiant come with a version of Q3Map2 installed, though it isn't the latest version. I would recommend downloading the latest version from shaderlab and using that instead (see my signature). Don't install the new Q3Map2 over the old one in the Radiant directory, install it in a new directory... I think some of the lib files are different. Run it with a batch file or download a front-end GUI like Q3Map2Toolz (Google it).
Generally, ASE's are better than MD3's:
-They are just a text file, so it's easy to edit them.
-MD3's need to be compiled from some other format (like from an ASE). ASE's just need to be exported.
-MD3's are limited to a bounding box of (IIRC) 1024x1024 units. ASE's are not.
-Quite a number of programs support ASE (or at least have plugins that do), so it's easy to use import/export from them.
Answers to some of your earlier questions:
-how do i export to MD3 with a texture/shader?
IMO, don't bother. Use ASE instead.
-can i convert an object to a brush in gtk?
If you mean convert a model to a brush... you can't. If you ever needed to do this, it would be easier to build the thing in GtkRadiant in the first place.
-can an object be converted to a map?
No.
-can you add gtk shaders/textures to an md3-object?
Yes, of course. See the tutorial linked above (I suppose you already know this by now).
BTW, don't refer to GtkRadiant as "GTK". GTK stands for Gimp Tool Kit, which GtkRadiant is based on - but GTK and GtkRadiant are not the same thing. If anything, refer to it as Radiant or GtkR or something.
[size=85][url=http://gtkradiant.com]GtkRadiant[/url] | [url=http://q3map2.robotrenegade.com]Q3Map2[/url] | [url=http://q3map2.robotrenegade.com/docs/shader_manual/]Shader Manual[/url][/size]