var damage: float = 15.0;
var playerStatus : Player_Status;
var damageDelay = 3.0;
private var lastDamage = -10.0; //Don't touch!!    

function OnTriggerEnter()
{
    playerStatus = GameObject.FindWithTag("Player").GetComponent(Player_Status);
}

function Update()
{
    if(playerStatus != null){
        if(Time.time > damageDelay+lastDamage){
            playerStatus.ApplyDamage(damage);

            lastDamage = Time.time;
        }
    }
}

function OnTriggerExit()
{
    playerStatus = null;
}

There you go, use a simple timer.

Maybe making a function doDamage and calling WaitForSeconds? As you want 5 of damage every 3 seconds you can do:

if(playerStatus != null) {

doDamage();

}

function doDamage() {

yield WaitForSeconds(damageDelay);
playerStatus.ApplyDamage(damage/3):

}

When it reachs a certain amont of damage you can turn like a bool to false to lock the doDamage function.