This has been formalized in a triangular graphic. Why not provide this to a project manager, on a corkboard or magnetic board, along with pins/markers to show original plans and current estimates ?

Doesn’t non-fast imply non-cheap?

"Cheap" == billable time * per-worker capital and support, not elapsed time.

Obviously, if a given workforce on a time-based compensation consumes more billable time units, they will cost more. That isn't the general case envisioned by the GFC triad, but rather that additional resources (bodies, brains, greater talent, tools, extended workdays or weeks) be applied. Classically, The Mythical Man Month. Accrued technical debt is another (often hidden) cost.

Good / Fast / Cheap is a possibility / optimisation frontier. When you are at the frontier, you cannot optimise any one third factor without sacrificing one of the other two.

An obvious countertactic in negotiation is to argue that the task is not yet at the frontier / optimum. In practice, this may be true, though the frontier may still lie far closer than such tacticians generally seem to posit.

If I understood your question, cheap + good + slow is generally when you do everything yourself.

I guess it depends on your salary. ;)

The way it makes more sense to me is when you spread out the same amount of effort over longer elapsed time, quality increases just my not rushing things and being able to reflect more.

It can imply junior engineers/interns doing the work as well hence cheap and slow.

There's another dimension which is "scope" and is also often negotiable.