I did some eyeballing and calculations...

Quake unit to meters is a factor of ~20

eg
1000 u = 50m 

Not sure if I need to post here or on "Coding help", I hope it's ok in mapping help.
I have the following:
-1 trigger_counter with target "mdt", targetname "zt", count 3
-1 trigger_relay with target "ds13", targename "mdt", message "Door Unlocked", delay "2"
-3 monster_zombie, each one has target "zt"
-1 door with targetname "ds13", message "Lockdown"

This entire setup works, with some small (but very annoying) problems.
When I try to open the door, the message "Lockdown" is shown, that's perfect.
When I kill a zombie with my weapon (quad shotgun or rocket launcher for example), the message "Only x more to go..." appear.
When all 3 zombies are dead, "Sequence completed" is shown, then, after 2 seconds the message "Door Unlocked" appears and the door ds13 is opened.

My first problem is that if I kill the zombies with a machine (for example a func_door that will crush the zombie), the message "Only x more to go..." doesn't appear.
Even worse, in the end, when the last zombie is killed by crushing it, "Sequence completed" doesn't appear, and also "Door Unlocked" doesn't appear, but (this is nice) the door unlocks.
Why doesn't the messages appear, and what can I do?

The second problem - after the door is opened it stays opened, is there a way to make it close, but be "unlocked"? 

Hi RaverX. I'm going to tackle those in reverse order. To "unlock" your door, set it up as a door opened by a trigger_multiple. Then use the approach in https://tomeofpreach.wordpress.com/2012/11/18/mid-map-entity-spawning/ to convert the trigger_multiple into one that spawns mid way through the map. Spawn the trigger when all the zombies are dead.

The issue you're having with trigger_counter is best understood by thinking about co-op. In that arena, trigger_counter messages are not broadcast to all players. Some of the time that's desirable: if the players are off in separate parts of the map, both counting down their own trigger_counter, displaying the other player's progress will just confuse matters. On the occasion where the two players are in the same area you might wish it was otherwise, but the design is locked in.

What that means is that the game needs some way to know which player did a particular thing, so that the counter messages can be delivered to just that player. Your problem is that the zombies aren't being killed by a player at all - because the machine killed the zombies, the message goes to the machine!

If you were really insistent that you must have the messages display, and you don't care about the effect on co-op, you could use the hack from earlier in the post to work around it. Create one spawnable trigger_once the size of your entire map for each zombie. Have each zombie death spawn one of the trigger_once entities, and make them target the trigger_counter. Each time a zombie dies, the trigger_once will be created, and the player will instantly touch it. The action that advanced the counter now comes directly from that player, so the message goes back to them.

It's a lot of trouble and pretty hacky, but it would work. If you did, there's a nice extra little flourish you could add. Give each trigger_once a slightly different delay like 0, 0.05, 0.1 etc, then simultaneous kills would be staggered on the trigger_counter, so you'd see each number pass by. 

Thanks, Preach, at least now I understand the problem.

I don't like the approach with a huge trigger, but you gave me a few ideas. 

Hope this hasn't been asked before, but I've been Googling for like 30 minutes so it might just be time to ask. I've been trying to make a Quake 1 map in TrenchBroom for Arcane Dimensions which takes place mainly on a giant elevator, and whatever entities I put on top of it seem to just not receive any light, even though the brush itself is being lit just fine. So if, say, I've got a point_light angled straight down over an item_health, the func_train underneath it receives the light, but the item_health remains entirely pitch black. Is this something that can be solved, or just an unfortunate quirk of working with func_trains? And if so, is there any kind of hack I could use to get around it? I've wanted to make this big elevator map ever since I first played the finale of ad_necrokeep, and I remember there being a normal-looking armor2 on the platform when that section starts, but I've had no such luck. (For what frustration this problem has caused me, I will say that fighting pitch-black monster_skullwiz's looks very cool.) 

The entities take the light value from the "ground", which is the worldspawn (level's static geometry). Stuff like trains, doors, buttons (func_#### and similar brush models) aren't considered "ground" because they aren't worldspawn.

This video should help:

https://www.youtube.com/watch?v=u8-mw79NGDU