Service desk escalaton

Answer:
Service Desk is, actually, heavily involved in Incident Management process. In Incident Management - there are two kinds of escalations:
- hierarchical
- functional.
What you are mentioning is - functional. That means - to the group with more expert knowledge. However, there is no requirement i.e recommendation that there must be e.g. three levels of escalation. Adopt it to your own organization. You can have more, as well as less than three.
Read the article "Incident Management in ITIL – solid foundations of operational processes" https://advisera.com/20000academy/blog/2013/05/21/incident-management-itil-solid-foundations-operational-processes/ to learn more about it.