Why do some elements on the periodic chart have three letters as a symbol, all starting with a u?

These elements are synthetic elements in that they do not occur naturally and only as a result of being made in a scientific laboratory. Elements are given the Uu(x) name as a placeholder until IUPAC (International Union of Pure and Applied Chemistry) approves an actual name for the element. This is under a process called "systematic element naming" (feel free to give it a google). The IUPAC grants the discoverers of the new element the right to suggest the name for that new element, so you may find that those weird Uu(x) elements have an actual name soon!!