You miss the fourth point of the triangle - cost, which (within the bounds of scalability and technology) means you can throw money at the problem. So it's actually a tetrahedron.

I have that tetrahedron on my wall. I call it "Pick 3". It feels like one of the laws of thermodynamics. Pick the 3 things that are most important to you, and the fourth is then fixed.

So:

If you want to deliver with a defined level of quality, to a defined scope and for a specific budget, then the time required to do this is fixed - you can't change it; or

If you want to deliver with a defined level of quality, within a certain timeframe and for a specific budget, then the scope you can deliver is fixed - you can't change it;

And so on...

For the mathematicians T = f(Q,S,C) or S = g(Q,T,C), etc. where these mythical functions f() and g() are unique to your team, project, organisation, methodology.

Your job is to change the equation.