Seems like part of the question is missing. Why specifically three dimensions of what? Dimension means: a measurable aspect of something; and it's a conceptual

Seems like part of the question is missing. Why specifically three dimensions of what? Dimension means: a measurable aspect of something; and it's a conceptual tool we use for things. If we're talking space (defined as a continuous area or expanse which is free, available, or unoccupied), we can define and measure length, width, height. So 3 dimensions. If we're talking area, 2 dimensions. If we're talking universe, we can measure aspects of other phenomenon (such as relative movement of objects). You could combine it with space, and could say we live in a 4 dimensional universe, 3 comprising space, 1 time... and call it: spacetime. Do 4 dimensions accurately model all that's observable in the universe? That depends on what you consider as the necessary aspects that make up the universe. Anyhow, in short, why 3 dimensions? Because it can be modeled that way and seemingly makes for a good model.