At HerdingCats, there is really good posting of the "iron triangle" described as the "3-body-problem". I recommend you click over and read it. (And, that's where I got the swell graphic)
But, not so fast!
But -- there's always a "but" -- the triangle famously leaves out quality as an explicit trading variable, and also "effectiveness-for-business-value" -- the PMBOK dropped the triangle some years ago -- after all what's the point of a doctrinally successful project if it's the wrong thing to do vis a vis business value? Actually: There's no point in such a project.
This is true whether in public or private sector, where in the public domain, business value is often "mission").
And, by corollary: is it all bad to be off according to project doctrine, but spot-on re the business? I think not.
So, perhaps it would be best to all spring for quality and business value as a five-element system.
On the other had, it's a great graphic. Looks like an independent suspension apparatus, which I suppose what it's supposed to be: independent trading variables.
Ooops! They're not independent: As a general matter in most practical projects there are correlations between them -- not always strong correlations; perhaps not even material correlations, but nonetheless, not truly independently sprung.
Thus, we can expect some compression/expansion of the springs without necessarily affecting the other balls. And that, of course, is the point of such an architecture -- isolation and pseudo-independence. By the way, the automotive guys have been using such independent suspensions for years, and to good effect.
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