I love your comparison between PM and a job in the circus. In essence, the PM job is to balance project constraints, like schedule, budget and scope, as you mentioned.

However, there are more project constraints that a PM must balance. Among them are risks, stakeholders’s interests, quality etc. In my opinion, the traditional triple constraints are used mostly just in project management softwares, like MS Project, to communicate the basic information with customers. In the real world, the number of constraints is much larger and depends on project

Son Nguyen
http://pmreviews.org