The triple constraints provide an interesting approach that could be used to help keep a project on track. In simple terms, a project management practitioner needs to ensure that projects are delivered within budget and on time, meet the agreed scope and meet the defined quality requirements.
A simple example can be useful to explain how these constraints impact decision-making. Suppose you are releasing a product on schedule, a competitor has released their product and now you are under pressure to deliver before the planned date. In this case, the schedule needs to be pulled forward, impacting the other constraints. Most likely, the cost will go up because you will need more resources (although sometimes adding more resources can actually lead to delays).
Alternatively, you could reduce the scope of work, such as not deliver non-critical features and still deliver within budget and cost. This simplistic example shows how project stakeholders need to be aware of the triple constraints and the impact. The project manager needs to juggle these constraints as per stakeholder expectations to deliver successfully.