How? Simple. You're making a 3D model with animations and then exporting it with MD3 extension. Then apply the model to your weapon and set the animation frames in states.
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I never exported animations with blender but there is a M3D Plugin, i use it for static meshes. For the animation part, i bet there are some Tutorials for Quake 3 on Youtube. In the "Workarounds" Thread is a link to the Plugin i used and some instructions for static meshes.
For the animations part in engine, you have to look at the russian Manual and google translate it. Look at the Stuff about "states" Its a bit confusing at first but not to hard. At least the stuff i looked up.
As i said, i never made animated 3D models for this engine, but as i remember its all in the russian Manual and the Wiki.