Ok, as I see from your message you are pretty new to map making so welcome to world's editor
Now I really suggest you to just try out stuff, even without any goal just to familiarize your self with the editor.
Anyway back to your question.
This will need 2 triggers, one to make the quest (you can put in it all the quests you want, not only this one), and one to update/finish it.
First trigger:
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Unknown
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Events
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Conditions
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Actions
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Quest - Create a Optional quest titled Kobold Caves with the description You need to destroy..., using icon path ReplaceableTextures\CommandButtons\BTNAmbush.blp
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Set KoboldCaves = (Last created quest)
This trigger makes the quest.
Second trigger:
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Unknown
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Events
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Conditions
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(Unit-type of (Triggering unit)) Equal to Footman
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Actions
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Set Buildings = (Buildings + 1)
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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Then - Actions
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Quest - Display to (All players) the Quest Update message: 1/3 kobold caves de...
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Quest - Change the description of KoboldCaves to You need to destroy...
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Else - Actions
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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Then - Actions
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Quest - Display to (All players) the Quest Update message: 2/3 kobold caves de...
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Quest - Change the description of KoboldCaves to You need to destroy...
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Else - Actions
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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Then - Actions
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Quest - Display to (All players) the Quest Update message: 3/3 kobold caves de...
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Quest - Change the description of KoboldCaves to You need to destroy...
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Else - Actions
This is a little harder since as you appointed, it uses 2 variables.
The first called KoboldCaves which we made in the first trigger, is a Quest variable.
You will need to make quest variables only in case you have (and I presume you will have) more then one quest.
The second variable called Buildings is a unit variable indicating the 3 buildings.
Basicly what this trigger does, is just writting a quest update on the screen and in the quest log, when the players destroy the buildings.
It can be done in various ways, I this this is the shortest one.
The continuation actions... you make them