In one, she outlines how she struggled to come to terms with the prank call by Australian DJs Mel Greig and Michael Christian to the hospital where a pregnant Kate was being treated for severe morning sickness.
But in another she criticises senior colleagues at the King Edward VII hospital over her treatment after the pair had pretended to be the Queen and Prince Charles asking about the duchess's condition. Her husband Ben Barboza wants an inquiry into the hospital.
The third letter is more practical, dealing with her funeral arrangements. Scotland Yard detectives are examining the notes.
Meanwhile, death threats have been made against the staff of the Australian radio station whose DJs made the prank call. About a dozen staff of Sydney-based 2DayFM station have been moved into safehouses and up to 10 executives have been assigned bodyguards following death threats.