The much better instruction is “pick a point on the triangle”. If you want it as good as possible, it’s neither cheap nor fast. You can have kinda good and kinda cheap, but it won’t be fast at all. And so on.
When you're a plumber talking to a homeowner, or any human talking to any other human... seriously triangle diagrams and pick a point in 2d space along 3 axis...? It's a valid graph but it does not roll off the tongue and does not convey the point as simply and quickly. It just adds granularity and complication to the same essential concept.
That next level of refinement maybe comes later in the same conversation, but the shorter sound byte still comes first and is not wrong just because it's the outline rather than the full dissertation.
E.g. waiting until someone else makes something so you don't need to make it yourself, waiting for some new technology to become cheaper before jumping on it, waiting until an opportunity cost isn't as high, etc.
Good fast cheap: pick one.
For example: unskilled labour used to be essentially free. In that case good and cheap meant either training someone (slow) or finding someone who already did it as a hobby (slow because they are hard to find then don’t prioritize your project).
Similarly: the culture it was created in didn’t value “personal time” so doing it yourself over time with a minimal cash budget was considered cheap. Now we’re all pretty familiar with opportunity costs and paying ourselves for sustainability.
Personally, while I feel like this triangle does show it’s age a bit it still serves as an excellent reminder when Those In Charge are trying to justify convincing themselves that they can have all three. Thankfully just that checkin has multiplied more than a few invoices so that there’s enough budget for quality.