take one of the working alarms off of the other 2 units and hook it to this one and see if the problem persists.
If the problem goes away then put a new alarm on it, the one that is on it is bad.
If the problem still occurs with the other alarm hooked to it then the problem is in the circuit not the alarm itself and you should start looking for bad grounds or possibly a bad manager card since it controls the signal output for the alarm.
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