[Joeri]

You can have all three. Prioritizing quality from the start, by doing design reviews, pair programming, tdd and so on, will actually reduce time needed to deliver fully functional software, and therefore lower overall cost.

[icebraining]

reduce time needed to deliver fully functional software

Except you can deliver a cheaper product faster if it's not fully functional, hence the triangle.

[dimino]

In theory.

In practice? Pick a spot inside the triangle.