I think because the number 3 is still pretty low it isn't so suspicious to people, but imagine we lived in some universe with, say, 186 dimensions (that miraculously supported intelligent life, so there's someone to ask the question - I know about the anthropic principle). Wouldn't we have asked ourselves something like "Hey wait a minute, 186 seems pretty arbitrary, what's so special about this number that it should be the number of dimensions in our universe?". I know that string theory says there's 10 or 11 dimensions (or was it 26?), and space-time is really 4d not 3d, so let me generalize the question. What made the universe have the number of dimensions it happens to have, whatever this number happens to be? I'm wondering if maybe there was a dimension-making mechanism, like the Higgs mechanism is supposed to create mass.