First of all zammad Is great tool , Thanks zammd community
Question - I want to re-escalate new ticket
If ticket is not attended by Agent-A
then send escalation email to Agent-B
if ticket is not attened by Agent-B
then send escalation email to Agent-C last person
You might want to try the schedulers instead of the trigger, as triggers only fire if a user based update occured to a ticket. Escalation changes of a ticket do not count in these cases.