JFK and LBJ, for example.

The three-letter thing tends to happen when the middle name or initial is ubiquitously used, and when the resulting three letters are distinctive and catchy. Thus, no JEH (J. Edgar Hoover) and no HRC (Hillary Rodham Clinton), but sometimes GWB (George W. Bush). However, this suggested an even shorter nickname: simply W (where HW was his dad).

If you could pronounce IXK, he'd be a good candidate (though he's not a politician (yet)).