furrycat's 3D Studio MAX level editing tutorial
What you need
Of course you need 3D Studio MAX 2.5. If you haven't got this you might as well stop reading right now. In fact you can use 3D Studio MAX 1.x or 2.x as well for the purposes of this guide but releases 3 and onwards will not work. Release 2.5 is the best one to have since you can use it to make character and weapon mods for SWAT3 as well.
Do not ask me where to get 3D Studio MAX. Your question will be ruthlessly ignored and probably forwarded to other members of the SWAT3 modding community so they may laugh viciously at your stupidity. Either you have a legitimate version of the software or you steal a warez version. In the former case there's nothing more to say. In the latter case it's your business. I won't help you rip off the publishers.
The key to the whole game is the Quake MAP exporter plugin which you can get from http://conitec.net/qmapexp.zip. The .map file format used by Worldcraft is merely a development of that used by Quake 1 all those years ago. This plugin lets you write Quake map files.
You also need Worldcraft, correctly configured for SWAT3. Yes that's right, you still need Worldcraft. This is because the levels 3D MAX spits cannot be parsed by Sierra's map compiler. Importing them into Worldcraft and then exporting them solves this problem. Furthermore, you may decide that creating entities in MAX is just too much hassle. In this case you'll want to concentrate on making the world geometry with MAX and letting Worldcraft handle entities. This is purely a matter of personal preference.
You'll need Sierra's map compiler (map.exe). It comes with Worldcraft so there's nothing to worry about there.
Finally, a half-decent text editor will be very helpful. One that can run macros. This is because you will have to remove bits of cruft from the files MAX exports prior to loading them into Worldcraft.
What you can do
With the Quake plugin you can create world geometry and place entities in the scene. Coincidentally, this is what Worldcraft allows you to do. Creating map geometry is a simple case of slapping some blocks ("brushes" in Worldcraft-speak) about the place and assigning materials to them (more on that later). As far as entities are concerned, you can use the plugin's Quake 3 helpers to position your entities, and edit their attributes directly. Unfortunately, the plugin's entity definitions are hardcoded for Quake. You will need to study the correct entity types yourself. Check out Sierra's Mod HQ FAQ and/or attempt to make some test maps in Worldcraft for more details.
It isn't too late to turn back
I told you it wouldn't be easy. I told you it would be a hassle. If you're sure you want to continue, read on.
On to part two: creating geometry.