The title is the question. But I don't really know why our real world has 3 and not more or less dimensions. Are there some mathematical considerations about this fact?
String theory uses some quantum mechanical computations and has found out that the critical dimension is 10. If 6 dimensions are compactified one has 3 dimensions plus one time dimension. The 6 compactified dimensions mathematically should induce our 4 fundamental forces of nature. However, I don't really think that String theory is true.
Are there different kind of explainations of this fact?