3. The scope, budget, and schedule must remain balanced throughout the project.As a rough rule of thumb the 'iron triange' is a useful aphorism, but it is not good business to take just a 'cost' view of a project. Any project is an investment of resources and effort over time; the investment has to give a return to the investor that is better than alternative investments. The question is investing to produce business value through achieving the required performance (or delivering the required capabilty).
In project management there is a key concept called the triple constraint or the project management triangle. The idea is that while managing a project the project manager must keep three constraints in balance; the scope or work that is required to produce the projects end results, the amount of time required to perform the work, and the cost or budget for the project. Scope, time and cost are almost always competing constraints. Typically, you cannot change one constraint without affecting one or both of the other constraints. The key principle is that as change happens, you must keep these three constraints balanced.
Sure, any project decision will have implications for expenditure, delivery and performance, but 'scope', 'budget', and 'schedule' is only part of the story. The project manager has to manage the whole story, including the value sought to produce the investment return. The PM has to be concerned with the outturn value of the project as a result of the delivery efforts.
So, I prefer to frame a project as investing (cost) to deliver (schedule) capability (scope) to maximise return (value).