Musket missing something

Right. I've worked out that it is me you're calling a "Dozy sod". You've been spending too much time with Steve Shaw.

The relative position between three objects and allowance for movement through time is pure three dimensional.

Not under General Relativity it isn't. It isn't about time dilation due to high velocity, it's about the curvature of space in the presence of massive bodies.

In applied science, Newtonian mechanics is fine for all ordinary, everyday purposes like playing billiards, jumping off tall buildings or sending a spaceship to Mars. It might give you problems with Mercury because, being so close to the Sun, its orbit does not follow Newton's predictions but it does follow Einstein's.

The clock in your sat nav is deeper in the Earth's gravitational well than the clock in the satellite so runs slower. If the programme didn't allow for this there would be an error of something like ten miles. Newton's theories are very, very good but there are real life situations where they give the wrong answer.

As a theory of pure science, General Relativity gives an entirely different description of how the Universe works. It supersedes Newton.