Tuesday Night Trivia

Yet another question involving the initials of sovereign nations:

By my count, the English-language names of 25 sovereign nations begin with the letter "S." (Southern Sudan will make 26.) "S" is by far the most common first initial. Only three other letters begin the names of 15 or more countries. Name these three letters.

Note/Hint: The exact number of nations whose names begin with these letters depends on which name you use for one of the countries. It has two, both of which begin with one of the letters in question.