OK, I've made a few small tweaks based on dobugabumaru's comments:
MAP03: Another neat level, with it's most interesting idea being the goodies etched into the wall. Thoughts: At the start, the ASHWALL above the "entrance door" is raised and it looks a bit weird/out of place. In the room with the "POISON" wall warnings the corner near one of the doors is lit up, when it should probably be dark. Also when you return to that room with the red key, triggering the ambush, it is almost guaranteed that the HK and caco will fight, nullifying any sense of real danger.
The room past the red key door is pretty cramped and dangerous, which is always good. On the way to the blue key, the specter can't get down the stairs, so he's a non-threat. The monsters that come out afterwards are a nice surprise, but the following double HK surprise with a pinch of Baron is just too obnoxious to battle with solely the SG and chaingun (baron could easily be replaced with just another HK). The SSG and RL are surprise secrets, so it kinda balances out, although I don't like the idea of having the SSG as a secret. Also, health is nicely balanced for the first half of the level, but it's completely missing near the end which is another red flag for me (since you'd ideally want to give the consecutive players as close to 100% for the next level). Again, health is given out through secrets, so maybe it's just a preference thing. Otherwise it was pretty fun, although ammo was quite superfluous compared to the first two maps (you can exit with like 80 shells; two boxes of shells in the red key room are unnecessary and it's generally a bad idea to overload goodies in secret rooms, like the backpack secret).
Oh, and visuals were okay. The DOORTRAK for the crushers in the poison pathway is a nice touch.[/b]
-I removed the Caco from the ambush after getting the red key. You're right, he is a non-threat thanks to the Hell Knight.
-I changed the Spectre into a group of Imps.
-I changed the Baron into a Knight.
-I tweaked the appearance of health packs near the end.
-I removed one of the Boxes of Shells from the Red Key room.
-I tweaked the ammo power-ups in the Backpack secret.
Here's a new version: http://www.mediafire.com/?161bfqidxr71ej5