the three Rs
R R, the eighteenth letter of the English alphabet, is a vocal consonant. It is sometimes called a semivowel, and a liquid. See Guide to Pronunciation, §§ 178, 179, and 250-254. “R is the dog's letter and hurreth in the sound.”
Note: In words derived from the Greek language the letter h is generally written after r to represent the aspirated sound of the Greek "r, but does not affect the pronunciation of the English word, as rhapsody, rhetoric.
The English letter derives its form from the Greek through the Latin, the Greek letter being derived from the Phœnician, which, it is believed, is ultimately of Egyptian origin. Etymologically, R is most closely related to l, s, and n; as in bandore, mandole; purple, L. purpura; E. chapter, F. chapitre, L. capitulum; E. was, were; hare, G. hase; E. order, F. ordre, L. ordo, ordinis; E. coffer, coffin.
The three Rs, a jocose expression for reading, (w)riting, and (a)rithmetic, -- the fundamentals of an education.