So there are a number of ways to do this.
1 - Create a true/false field called "Escalated to L3" and then add a BR that checks the box when it goes from L2 to L3.
2 - Create a Metric that will track this and then run your report on the data generated from the metric.
3 - Something else I have not thought of yet.
Option 1 is probably the easiest option but this is adding a field per escalation type you have which can get messy.
