But there are useful physical theories that do not have three dimensions. True, those theories are usually related to a “fundamental” three space/one time dimensional theory, but notions of what is “fundamental” have changed over time and surely will continue to do so. To me, it takes a leap into metaphysics in order to say that the world “really is” three-dimensional, as opposed to saying that describing the world as if it were three-dimensional is useful.
It’s fair to say that the world really is three-dimensional, if by “the world”, you mean the world we perceive. It might not be fundamental, whatever that means, but that doesn’t make it less real.
I don’t believe in metaphysics. I don’t even believe that other people believe in metaphysics.
What I mean by that is no matter what you may argue on a message board, you and everybody else who is compos mentis spend every waking minute of every day treating the world as if it really, really, really had three dimensions. Not merely a useful fiction, but real. It is the ultimate distinction without a difference.