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If the idea is that this creature is the third upgrade, then you can make it so that a third-level building needs to be constructed to hire it.
To do this, simply remove the second-level archer upgrade from the second-level building's available creatures. Then, create a third-level building and only list the second-level archer upgrade there.
Next, in the file for the most basic archers, remove the second-level upgrade.
In the file for the third-level archers, make it so that they reference not the basic archers but the first-level upgrade in the "base creature" field.
In the file for the first-level upgrade, add two upgrades in the upgrade field. The first can be anyone, and the second should be the third-level archers.